The accuracy of balance formula is very important for wind tunnel test. Traditional balance formula is an incomplete quadratic polynomial. Based on the calibration data of a balance, the coefficients of the polynomial are obtained by the least square method. Terms of the incomplete polynomial are specified in advance. However, some abandoned terms may affect the result’s accuracy, and some reserved terms that have very small coefficients may be false terms. Those become the source of calculation errors. With the increasing demand for data accuracy in wind tunnel tests, the traditional balance formula is incompetent. To improve the accuracy of the balance formula, a method of establishing new balance formula based on Genetic Algorithm has been proposed. Taking a complete polynomial as the initial balance formula, a new balance formula can be obtained by optimization calculation with some "discard term" trips. Based on the calibration data of a six-component strain-gage balance, the accuracy of the new formula and the traditional formula is compared. Results show that the new formula has higher accuracy.