Experimental study on the energy transmissivity of high -power nanosecond laser pulse focusing in air versus input laser energy has been carried out. There are three stages: when the input laser energy is relatively low，all of the laser energy can pass through the focusing point; when the input laser energy increases gradually，energy transmissivity decreases steeply from nearly 100% to lower values; when the input laser energy keeps on increasing，the transmissivity decreases slowly further. By defining the critical electron density and the corresponding critical time point，successful theoretical analysis has been carried out for the above experimental phenomena. When the free-electron density is less than the critical value，multiphoton ionization plays the dominating role; when the free-electron density is higher than the critical value, the inverse bremsstrahlung process dominates. The critical time point is the time when laser pulse reaches critical electron density in the process of laser pulse passing through the air; its position on the laser pulse is determined by the input laser pulse energy，meanwhile this position determines the energy transmissivity of the laser pulse. So we can calculate the critical electron density by measuring the transmissivity versus input energy and comprehend the characteristics of transmissivity for laser pulse focusing in the air.