Abstract: The residual stress of metal components in the process of processing and service causes deformation and even stress fatigue, which seriously endangers the safety of the key structure. In order to avoid the effect of temperature and measure internal stress gradient at crucial components, a differential four-channel ultrasonic stress detector was designed based on acoustoelastic effect of longitudinal critical refraction wave. The detector was designed based on field programmable gate array (FPGA) and mainly consisted of ultrasonic transmitting and receiving circuit, signal processing circuit and operation software. The detector equipped four-frequency ultrasonic transmitter probe and two broadband receiver probes to eliminate coupling errors caused by temperature and probe replacement. The stress measurement of aluminum and titanium alloy structures was carried out by standard tensile test, and proved the capability of four-channel measurement of stress.