A W-band Single Pole Double Throw(SPDT) switch with low insertion loss and high isolation that based on Positive Intrinsic Negative(PIN) diodes is presented. The finline structure in the form of comb-line high pass filter and an improved Y-shaped junction are introduced into the circuits to increase the isolation and reduce the insertion loss. Simulated results show that in the range of 88 GHz~99 GHz, the insertion loss of the SPDT is less than 0.7 dB, and the isolation is more than 58 dB. In the range of 90 GHz~95 GHz, the insertion loss at output port 1 is less than 3.7 dB, and the isolation is greater than 33 dB, while at output port 2, the insertion loss is less than 3.8 dB and the isolation is greater than 33 dB, which are indicated by experimental results.