The invention relates to a measuring method for linear losses inside a cavity of an all-solid-state laser. A pumping source (1), a gain crystal (2), ring resonant cavity, an isolator, a beam splitter (6), a first power meter (7),and a second power meter (8) are arranged, wherein the isolator is formed by a magneto-optical media (3) and a half-wave plate (4) that are arranged in a permanent magnet. The measuring method is characterized in that a frequency multiplication crystal (5) having a non-linear effect is introduced into the resonant cavity, wherein a phase matching way of the frequency multiplication crystal (5) is an I type or II type non-critical phase matching way; output values of corresponding fundamental waves and second harmonics of any two working temperature points of the frequency multiplication crystal (5) are measured in a single frequency area; and then the two groups of numerical values are substituted into a linear loss expression inside a cavity to carry out calculation, so that a linear loss value inside the cavity of the laser as well as a numerical value of a pumping factor is obtained. The linear loss expression includes a non-linear transformation factor of the frequency multiplication crystal (5), a fundamental wave output power, a second harmonic output power, and a pumping factor.