In diode laser array (DLA) grating-external cavity spectral beam combining (SBC) system, the total effects of lens aberration, fabrication error of grating and‘smile’effect may lead to the light crosstalk between light-emitting units of DLA. Based on the analysis of generating mechanism of light crosstalk, the main kinds of light crosstalk can be divided into two categories, including the stray light returning to the light-emitting unit itself and other units due to the feedback of external cavity, resulting in the self-oscillation and coupled oscillation. On the above basis, the diode laser rate equation involving light crosstalk has been put forward, and the combining efficiency of DLA emitters has been derived. Furthermore, the impacts of the self-oscillation and coupled oscillation on the combining efficiency have been analyzed. Results indicate that the two categories of light crosstalk involving both the stray light returning to the light-emitting unit itself and the stray light injected into other units decrease combining efficiency of light-emitting unit to different extents, and the impact of the latter is larger. In practical applications, some methods should be taken to control the light crosstalk.