By using Mach-Zehnder interferometer, we gain time series interference patterns of delayed double pulse laser and three-monopulse laser produced air plasmas. And then we gain electron density values in the centre of plasma region at different moments. We compare electron density values of the plasmas produced, respectively, by delayed double pulse laser and three-monopulse laser. The results show that the electron density of the plasma produced by the delayed double pulse laser is greater than by the three-monopulse laser at the same time after the second laser effects. The electron density time change processes of the plasma produced by delayed double pulse laser and monopulse laser of the same injection energy are analyzed theoretically. The analysis results show that when the same laser energy is injected, the delayed double pulse method can increase the plasma existing time effectively.