The invention provides a welding penetration detection method for an aluminum shell of a lithium ion battery. The method comprises the following steps: 1) cutting a welding profile after an aluminum shell and a cover plate are welded; 2) polishing the welding profile; 3) immersing the welding profile into 0.01-0.5 mol/L NaOH solution for 1-30 minutes; 4) measuring the welding penetration of the welding profile after cleaning the welding profile. Compared to the prior art, the method of the invention can precisely measure penetration of the aluminum shell and the cover plate after laser welding, and plays a directive effect on actual production exercises, thereby gradually improving the welding quality and the performance of a battery cell.