The soft open point is a power electronic device that can realize the flexible adjustment of the distribution network. It usually adopts the controller hardware-in-the-loop platform technology to achieve diversified functional verification requirements. However, the inherent interface delay of controller hardware-in-the-loop will reduce the power angle dynamic accuracy of the transient stability test, which causes the test results are different with the actual system response. In this paper, the controller hardware-in-the-loop test system of soft open point is designed firstly, a time-domain mathematical model of the test system is established, and the influence of the delay process on the accuracy of the system transient stability test is analyzed. Then, the transient stability-based equal area criterion is used to quantify the influence of the delay on the accuracy of the simulation system. Based on the accuracy and simulation scale of the simulation system, the optimal selection and design of parameters for the interface delay is explored. Finally, the correctness of the theoretical analysis is verified by RT-LAB-based controller hardware-in-the-loop test system.