The GaN-based microdisk laser based on whispering-gallery modes (WGMs) has the characteristics of high power, low threshold and small size. It has a wide-ranging application prospects and is needed in many fields such as optoelectronic integration, biomedicine and data transmission. However, due to the lack of conductive substrates and poor material grown quality, GaN-based microdisk lasers have long been dominated by light-injected microdisk devices. To achieve an electrically-driven GaN-based microdisk laser, a current block layer is typically introduced in the central region of the microdisk laser. This allows for injection of current only at the edge of the microdisk, resulting in that the low threshold laser output can be achieved, and the heat generation in the central region can be well suppressed. Although the effect of the central current block layer has been reported, there is a lack of sufficient reports on the effect of current block layer size on the microdisk laser and its design guidelines. In this paper, the effect of the gain distribution affected by the current block layer design on the performance of blue laser is systematically investigated. According to the structure of the reported blue microdisk laser, the device structure of this study is designed (Fig.1(a)). The gain material in the active region is set as the Lorentz gain material, the real and imaginary part curves of the refractive index are obtained by setting the material parameters (Fig.1(b)-(c)). The Finite-Difference Time-Domain simulation (FDTD) is used to analyze the effect of the gain distribution on the performance of the blue laser. The no-gain circle region represents the area where current is unable to inject due to the presence of current block layer Firstly, the performance of the microdisk laser is studied by changing the diameter of no-gain circle region, and the resonance of the device is simulated by FDTD (Fig.3). The results show that there are five distinct modes, all of which exhibit a gradually decrease in power as the diameter of the no-gain circle increases. In addition, the total power output exhibits a decreasing trend with the increase of the diameter of the no-gain circle region. Specifically, the total power first decreases slowly from 0 μm to 0.4 μm, followed by a sharp decrease beyond 0.4 μm (Fig.4). Therefore, when the diameter of no-gain circle is less than 0.4 μm, it will not significantly affect the output power and threshold gain of the laser. Secondly, by adjusting the offset of no-gain circle to align with device edge, the effect of space between no-gain circle and device edge on the performance of the microdisk laser is analyzed. The analysis results show that the powers of the five modes decrease as the no-gain circle gradually approaches to the edge of the device (Fig.5). And compared with the first-order mode, the peak power of the second-order mode decreases faster (Fig.6). Finally, the effect of edge defects on the performance of GaN-based micro laser is analyzed. The peripheral no-gain ring region represents the device sidewall defect area from the dry-etched process. The results show that the peak power of the five modes decreases rapidly with the increase of no-gain ring width (Fig.7). Moreover, when the no-gain ring width increases, the power of the first-order mode decreases faster than that of the second-order mode (Fig.8). This is due to the fact that the distribution range of the first-order mode is closer to the edge of the microdisk. In this paper, FDTD is employed to investigate the effect of different gain distributions on the performance of blue microdisk lasers. The optimal distance between the edge of the current block layer and the edge of the microdisk is determined to be 0.6 μm. Too large distance will make some current injected in the middle not contribute to the power of the laser, resulting in increased threshold current and heat effects. Conversely, if it is too small, it will increase the absorption of the microdisk laser, thereby reducing the power of the laser. Moreover, if the current block layer is offset due to lithography, it will result in an increase in absorption in device and a reduction in the laser power, especially for the second-order mode. If the edge no-gain region expands due to the increased sidewall defects, the laser power will be significantly reduced, and the power of the first-order mode decreases more rapidly than that of the second-order mode. The results in this paper have significant reference value for the fabrication and design of microdisk lasers.