Gravity acceleration is an important mechanical constant in physics. There are many methods to measure the value of gravity acceleration, and the simple pendulum method is often used in university physics experiments. However, the experimental principle only studies the motion of the single pendulum ball and ignores the influence of the size of pendulum, buoyancy, cycloid mass and other factors, so the measurement results are not accurate enough. The relative accurate formula of gravity acceleration and the corresponding synthetic standard uncertainty formula are obtained by correcting the system error that caused in the measurement of gravity acceleration by the simple pendulum method. Finally, the measured data are analyzed and calculated reasonably, and the more accurate experimental results are obtained.