In spectral beam combining systems based on a grating-external cavity, due to some factors such as the “smile” effect of the semiconductor laser array and the error of the optical components in the external cavity, the beam from one emitter transmits into the external cavity and then can return to other emitters, thereby forming beam crosstalk between the two emitters. In this work, in order to investigate the physical mechanism of beam crosstalk and the influence of beam crosstalk on beam properties such as locked spectra and beam combining efficiency, based on the optical feedback semiconductor rate equation, the beam modes that can stably oscillate in the coupling cavity are derived, and the coupling cavity oscillating model is built. With the consideration of the mode competition mechanism in the coupling cavity, the effects of different crosstalk between two emitters with different intervals on the locked spectra are analyzed in detail. The results show that crosstalk leads to the shift of the peak of locked spectrum and the generation of sub-peak. The crosstalk between two closer emitters has a more serious influence on the beam spectrum structure, combined beam spot, and combining efficiency. The combining efficiencies influencing the 1 ^st, 2 ^ndand 3 ^rdcrosstalk are 45.5%, 50.2%, and 63.8%, respectively (When there is no crosstalk, the efficiency is 80.1%). Finally, the results of the theoretical analysis are verified experimentally, and the experimentally observed spectra under the influence of crosstalk show phenomena such as peak degradation, peak shift, edge burrs, and side lobes in spectra, which are consistent with the theoretical predictions. Moreover, according to the simulation results and experimental observations, it is found that the crosstalk can be suppressed to a certain extent by increasing the spacing between emitters, and the Galileo telescope system is suggested to suppress crosstalk and optimize the spectral structure and beam combining efficiency. Compared with the Kepler telescope structure, the Galileo telescope does not have a real focal point, which can prevent the local power from being too high, thereby damaging the optical components.