Abstract: To further improve the precision and range of measurement micro-SF₆ concentration in air, the relative time-difference method based on ultrasonic testing is proposed. Only a pair of ultrasonic sensors is used in application. At first a difference of travel time in air and in mixed gas is measured and then the SF₆ concentration is calculated accurately. The proposed method has advantages of both time-subtraction method and time-difference method, which are precision and large range. Furthermore, it can be implemented easily in field programmable gate array (FPGA). Simulation results show that precision achieves 10^-5 and relative error is less than 1% by using the relative time-difference method.