In recent years, the diode pumped alkali laser (DPAL) has been rapidly developed as a new laser source. The master oscillator power-amplifier (MOPA) system is one of the most feasible engineering methods to achieve the high power scaling of DPAL. Comparing with traditional lasers, DPAL′s linewidths of both absorption and emission are extremely narrow. Therefore, the linewidth of a seed laser is one of the most important factors that effects the output characteristics of DPAL-MOPA. A theoretical microcosmic kinetic mode is established for an end-pumped DPAL-MOPA system. The linewidth of seed laser and the spectral distribution of emission cross-section are taken into account in the calculation process. The influences of linewidth of seed laser on the features, including the output power, output linewidth, extraction efficiency, gain factor, and pump absorption, are calculated and analyzed in detail. The research is helpful in construction of a power-scaled DPAL-MOPA system.