Abstract: In order to reduce the influence of the discrepancy between in site and calibration environment on the calibrated results， a vacuum leak calibration apparatus， which can be used to calibrate vacuum leak in site， was designed with the help of theoretical calculation.The modularization and split architecture was utilized in designing the apparatus in order to take-down and transportation easily. The calibration apparatus consists of pumping system， flow calibration system， vacuum leak joint system， output system， charge gas system， constant-volume and pressure measurement system， and bake system together， and this apparatus can adopt two methods such as constant- volume method and constant conductance method to calibrate vacuum leak. The expected calibration range of the apparatus in operational site is 5×10^-10～5×10^-5 Pa ⋅ m³/s with a combined standard uncertainty less than 10%.