The train line test is the a direct method to research on the aerodynamic performance of high-speed train, and The micro pressure sensor is usually used in test process. However, the sensor size will influence the flow field of the measuring point, leading to the measuring error. To solve the long-neglected problem, the paper establish the train aerodynamic model and the train-sensor aerodynamic model, then use the finite volume method to solve the equation of the turbulence model. the pressure value of tow measuring points can be calculated, and the functional relationship between speed and measuring error which is caused by the sensor size is established. The results show that the absolute error value of measuring point along the speed is quadratic when the train runs in the open air, and the maximum absolute error is proportional to the square of speed when it runs through a tunnel. The conclusion of the study could be used to correct the measuring data on real lines, therefore providing more accurate measuring data to research the high-speed train's aerodynamic performance.