To reduce the dispersion mismatch effect on FMCW measurements， a method of eliminating dispersion mismatch by the forward and reverse tuning of an external cavity tunable laser is proposed. When the external cavity tunable laser is combined with an optical fiber path， the system produces a dispersion mismatch effect， which is manifested as peak broadening and shifting of the target signal. The ranging value changes with the increase in the tuning bandwidth， resulting in measurement instability. To solve this problem， the paper studies the system dispersion mismatch characteristics of the external cavity tunable laser in forward and reverse tuning. The results show that the dispersion trend is a symmetrical distribution of the forward and reverse tuning. The paper establishes the dispersion mismatch model of the forward and reverse tuning system. On this basis， dispersion cancellation is realized by forward and reverse tuning of the external cavity laser. This method does not require calibration of the dispersion coefficient of the system in advance nor cyclic iterative compensation. A single measurement can complete the dispersion compensation for the system， which provides a way to improve the efficiency of dispersion compensation.