A novel trigger circuit of signal sampling was designed for developing modularized high-resolution compact gamma camera. The design idea is to transfer the peak position detection of the signal of gamma camera into high-precision measurement of pole-zero timing. The unipolar input signal is shaped by a full-pass filter into a bipolar signal. The peak position of signal is determined by measuring the polar-zero position while a trigger pulse is generated corresponding to the peak position. The width and delay of the trigger pulse are adjustable by mono-stable trigger circuits. The pulse is used to trigger the data acquisition system. The experimental results show that the performance of the trigger circuit is stable and reliable. It can provide a high-precision determination of the peak position and well meet the requirements of the high-resolution compact gamma camera.