In this paper, the "short-time" part of the linewidth of a laser beam has been considered with the fundamental equations which describe the behaviours of lasers. This linewidth is described usually by △vs=(8πhv(△v′)2)/P, (1) here △v′ is the half-width of the electromagnetic mode of cavity, v the beam frequency, and P the output power. The main results can be summarized in the following way: (1) The linewidth is raised mainly from the dissipative system coupled to the electromagnetic mode, and the contribution of the dissipative system coupled to the molecules is negligible. (2) In the single-mode approximation formula (1) is correct, only if 1/T2?1/(2π)△v′,1/T2 being the homogeneous linewidth of molecules. (3) If the inter-actions between the exciting mode and other modes in the multimode cavity are strong, then formula (1) will be generalized to the following: △vs=(8πhv(△v′)2)/P+(4hv(△v′))/P somefromn=(λ′≠λ)(rλλ′2/(rλ′),(2) here γλλ′ is the correlative relaxation coefficient and γλ′ is the half-width of mode λ′. In certain case the second term in formula (2) may be much larger than its first term. Therefore, it is one of reasons for the disagreement between the experimental results about the linewidth and the prediction from formula (1).