In active coherent beam combination (CBC), a single-frequency seed laser is often considered to be indispensable for improving the spatial coherence property and obtaining high brightness in the far field. Nevertheless, the single-frequency radiation restricts the output power of coherently combined fiber amplifiers, owing to low stimulated Brillouin scattering (SBS) threshold. The seed laser oscillating at different frequencies proves to have the potential in mitigating SBS effects in fiber amplifiers, and therefore it is able to increase the emission power of CBC greatly. In this study, the basic mathematical model is founded on the basis of the fundamentals of CBC, and the multi-wavelength two-channel CBC estimation formula is proposed on the calculation of some two-channel examples. The optical path difference is the key factor in multi-wavelength CBC. We propose the concepts of "controllable area" and "uncontrollable area" for the optical path difference. If and only if the optical path difference falls into the controllable area, desirable combination effects can be obtained by the active control. In the case of getting the value of optical path difference randomly, excellent combination effects can still be gained and the probability has a close correlation with the spectrum profile, which is inversely proportional to the number of wavelengths approximatively. The multiple control method is competent for the multi-wavelength CBC. In this method, the optical path difference is adjusted into the controllable area by large optical path controllers and then the phase of each channel is locked by precise optical path controllers.