Stiffness is one of the most important performance indicators of cars’ parts. Currently, most of the major automobile panels are made of big sheet metal, whose stiffness are relatively small, which lead to a lot of difficulties in measurement during practice. The high stiffness of fixed stiffness testing instrument with gantry structure makes the automobile panel measured precisely. The finite element model was established by ANSYS in order to study the effect of load at typical work locations on strength and stiffness, and the strength of testing instrument under maximum load was also illustrated. The results showed that the displacement of the load point and the work load performed the linear growth. When the testing instrument was imposed the maximum working load, the displacement of load point was less than the measuring precision. The strength and stiffness of the testing instrument met the requirements of operation.