In order to solve the problem of rapid plasma expansion and small fringe offset caused by peripheral plumes in the high vacuum environment, it was difficult to detect a single interference fringe pattern. The electron density distributions of the laser-induced aluminum plasma under 1.333×10-4 Pa and 1.333×10-3 Pa pressure were obtained by using simultaneous phase-shifting interference. At the same time, the expansion process of laser-induced plasma under a high vacuum environment was simulated numerically by using a 2-D axis ymmetric fluid dynamics model, and the two-dimensional distribution of electron density was obtained and analyzed for the reasons for the deviation of numerical simulation results and the improvement methods. The results show that the central electron density of plasma decreases to 1.4×1020 cm-3 at 50 ns. The numerical simulation results are in good agreement with the experimental results, which verifies the reliability of the model. This research provides some reference for the study of laser plasma under high vacuum.