The influence of mode mismatch on measurement in continuous-wave cavity ring-down technology was theoretically analyzed and experimentally studied. According to q-conversion of Gauss beam and mode coupling theory, the influence of light beam radius mismatch and phase front curvature mismatch on measurement was numerically simulated and experimentally confirmed as to general stable cavity with large Fresnel number. The results show that mode mismatch mainly affects the cavity ring-down signal output power rather than its ring-down feature when the cavity′s Fresnel number is large. Under the same case of mode coupling, a laser of higher power and a detector of larger focal area can improve the measuring accuracy.