The embodiment of the invention discloses an LED light brightness adjusting method and apparatus. The LED light brightness adjusting method and apparatus are characterized by determining the PWM duty ratio by counting the number of high levels between two times of chopper processing in a sinusoidal signal; and according to the PWM duty ratio, adjusting the LED light brightness. The LED light brightness adjusting method and apparatus can solve the technical problem that in the prior art, a silicon controlled rectifier light modulation circuit is utilized to adjust the conductive phase of the silicon controlled rectifier in a sine AC signal to change the brightness of an LED lamp, so that power output is hard to maintain stability; and the LED lamp is slow in the starting speed and is not uniform in flashing and illumination. The LED light brightness adjusting method includes the steps: detecting a sinusoidal signal which accepts chopper processing, and counting the detecting number of the high levels between the two times of chopper processing in the sinusoidal signal; according to the number of high levels, and the relation between the number of high levels and the duty ratio, determining the PWM duty ratio; and according to the PWM duty ratio, adjusting the LED light brightness.