Based on a great deal of test data, all empirical formulas for projectiles penetration into concrete have a limited domain in engineering. According to the principle of gray correlation analysis, a comparison data sequences are composed of multiply factors that influence the penetration depth. Meanwhile, the reference data sequences are also organized by the actual parameters. A common method for choosing the empirical formula of penetration depth is presented based on the model of gray optimal theory. The results of example show that the result of the gray correlation analysis is consistent with the experiment result, and the error of penetration depth is less than 5%.