Double-pulse laser-induced plasma has broad application prospects and development space in laser processing, detection of element, material removal and other fields, so it is of great significance to diagnose it. The time evolution law of plasma was obtained by two-wavelength interferometry, to study the effect and mechanism of plasma induced by delayed double-pulse laser. A dual-wavelength interference diagnosis system based on the Mach-Zehnder interferometer was established. It had ability to acquire interferogram of the double-pulse laser-induced plasma. By processing and analyzing the interferogram, the evolution law of plasma electron density with the delay time of double-pulse laser was obtained. The result shows that the effect of the second pulse laser on the plasma electron density first enhances and then weakens with the prolongation of the delay time between double-pulse laser. Among them, when the delay time of the double-pulse laser is 10 ns, the enhancement effect on the plasma electron density is the strongest, when 30 ns, the average electron density in the central region can reach 6.49×10¹⁹ cm^-3, which is 26% higher than that of single-pulse laser-induced plasma with the same energy. Meanwhile, the effect of delay time on the mechanism of secondary pulse laser was studied. The research results provide a reference for the optimization direction of double-pulse laser-induced plasma.