In order to improve the overall performance of measurement-device-independent quantum key distribution (MDI-QKD), the properties of photon number distribution can be optimized by using the modified coherent source (MCS), and a MDI-QKD system is designed based on MCS. The relationship between key generation rate and transmission distance is derived based on calculation method of global estimation. The full use of single photon information during privacy amplification calculation stage is ensured, and the more accurate lower limit value of key rate is obtained. The performances of proposed system and MDI-QKD system based on heralded single photon source (HSPS) are compared by simulation analysis. Results show that the transmission distance of MCS light source is increased by 9% than that of HSPS light source, and the system performance is improved effectively.