The pump light unabsorbed by gain medium is used to heat alkali vapor cell in a self-heated alkali laser with a mini vapor cell. A theoretical model of alkali laser under semiconductor laser double-end pumping is constructed, and the influences of the length of gain medium, the temperature of vapor cell and the linewidth of pumped source on the output laser of the self-heated alkali laser with a mini vapor cell are analyzed. The results show that, pumped by a common external cavity semiconductor laser, the W-class output laser can be achieved when the length of gain medium is 2 mm. The output power of the self-heated alkali laser can be improved with a high pump power. The research results offer the theoretical foundation for the self-heated alkali laser experiment, and expand the application of alkali laser with low power.