Based on two-photon absorption and pumped by a 778.1 nm dye laser, a 420 nm blue laser is achieved by using alkali Rb vapor as gain medium. Utilizing the energy level structure of Rb atom, blue laser mechanism and two-photon absorption process are analysed. By absorbing two 778.1 nm photons, Rb atoms are transitted from ground state 52S1/2 to excited state 52D5/2, then to laser up state 62P3/2 and ground state 52S1/2 populating inversion is fromed through radiating mid-infrared photon. The 420 nm blue laser is achieved through 62P3/2 to 52S1/2. The development prospect of laser diode pumped alkali vapor blue laser is expected.