In order to study the effects of alkali vapor ionization on the scaling potential of diode pumped alkali lasers (DPAL), a photocurrent method suiting for measuring the ionization degree of alkali vapor that pumped by a short-pulse laser has been established. By using the photocurrent method, ionization degree of cesium vapor was measured versus different power density of pump laser, temperature of alkali cell, and repetition frequency of the pump laser. Experimental results show that the ionization degree of cesium vapor is merely around 1% even if the power density of the pump laser has reached 3×108W/cm2 in the case of temperature of alkali cell is around 150℃ and pressure of helium buffer gas is about 9.33×104Pa with no heat effects. Moreover, the variation of ionization degree of cesium vapor is minor relative to the obvious increase of the number density of cesium vapor when the temperature of alkali cell rises from 122℃ to 163℃. These results indicates important signification for cesium DPAL scaling by increasing the power density of pump laser and the temperature of alkali cell.