The disclosure shows a control circuit for backlight, a control method and a liquid crystal display device. The control circuit includes a display driving chip used to generate a video frame data enable signal and a first PWM signal; a pulse width modulation control circuit for receiving the video frame data enable signal and the first PWM signal, and modulating a duty ratio of the first PWM signal under the video frame data enable signal control to get a second PWM signal; a backlight driving chip for receiving the second PWM signal, and controlling brightness of the backlight based on the second PWM signal to increase brightness of the backlight during a frame data disable period of the video frame data enable signal. Decreasing brightness in liquid crystal display device caused by leakage characteristic of thin film transistor is solved through the disclosure to achieve consistent brightness.