The a-Si thin film transistor ( TFT) used as room temperature infrared detector is studied. The channel current of a-Si TFT changes linerlywith the ratio ofwidth to length (W /L) ,which is proved by theoretic analysis and ex-periment respectively. According to the theoretic analysis and experiment, increasingW /L does not influence the tem-perature coefficient of current (TCC). However, largerW /L improved the detectivity evidently,which presents the ori-entation of a-Si TFT infrared detector design optimization.