To ensure turning-off the laser in the system of continuous wave cavity ring-down technology, a new method for modulating laser current, named as laser detuning is proposed. With this method, the resonant state between the laser and ring-down cavity is ended. Hence, the incident laser to the ring-down cavity can be turned off rapidly. This detuning method does not require external optical switches (such as the acousto-optic modulator) to turn off the laser source so the system is simpler than previous ones and the cost is lower than traditional systems. The continuous wave cavity ring-down system based on the laser detuning is designed. The operating principle and performance of various devices are analyzed. The characteristics of the detuning process are analyzed and the difference between the laser detuning and laser turn-off are compared. As a result, the turning-off time in the technology of modulating laser current is shorter than the traditional optical switches, and the exponential curve is more distinct.