The continuous environmental radiation monitoring is very important after Fukushima accident especially for the evaluation of the decontamination. A portable gamma camera system is designed and fabricated with a compact digital front-end detector. A front-end detector consists of an 12 × 12 GAGG-APD (pixel size = 2 × 2 × 5 mm3) array combined with 144 channel Time over Threshold based ASIC. All the channel is digitized in this front-end detector without ADC. The whole system is composed of a front-end detector, a pin hole lead collimator and FPGA based DAQ system. The total power consumption is about 600 mW and its operating voltage is 3.3 V. The point source(137Cs) was measured and successfully reconstructed with different source positions.