In this paper, an ultra compact cascaded microring filter consisting of two master rings with radius of 2.5 μm and two slave rings with radius of 1 μm is presented and studied theoretically. The filter with a very large free spectral range (FSR) of 206 nm, a deep extinction ratio of 23 dB, a high quality factor of 2.76×10⁵, and greatly suppressed spurious modes of less than 0.1 dB is achieved. The spectral responses of the filter are simulated by transfer matrix method, and the results show that this filter has a great potential of sensor application.