Laser current is a key parameter in laser direct-part marking symbols on a substrate. Different laser current can create enormous impact on bar code contrast, but the influencing regularities are unclear. In this paper, a new significance test method based on the multivariate nonlinear model was proposed to investigate the influencing regularities. First, the multi-element nonlinear stepwise regression model between the laser current, the laser line spacing and symbol contrast was established, and then the model significance test was employed to evaluate the influence between the two factors and symbol contrast. Finally, the influencing regularities were found by comparing the influence between the laser current and the laser line spacing. These regularities are that along with the laser current value increasing the influence of laser current on symbol contrast decreases and that the effect is much smaller and can even be neglected when the laser current reaches or exceeds a certain value. This certain value is 15 A in the Nd:YAG laser Direct-part marking symbols on the aluminum alloy experiments.