The application of the capability of low-temperature poly-Si TFT circuits for high resolution X-ray active matrix sensor is explored. The integration of poly-Si TFT circuits on the glass enables an easy connection for the sensor array with fine pixel pitch. A novel charge sensitive amplifier circuit employing poly-Si TFTs is proposed for the readout system of the active matrix sensor. It can considerably increase the circuit's immunity to the unavoidable threshold voltage variations of the poly-Si TFTs. The VTH mismatch effect of the TFTs, which results in the offset voltage of the charge amplifier, can also be suppressed by the proposed circuit. However, the noise arisen from the VTH mismatch can be even larger than that from the VTH variation after compensation. It reflects that, for higher bit digital X-ray image sensors, the mismatch effect of the devices must be properly taken into consideration.