The invention discloses an LED light modulation method and a light modulation device. The method comprises the steps: enabling the light modulation period of an LED to be divided into a plurality of sub-periods; enabling the bit weight of brightness data to be distributed to the plurality of sub-periods, thereby obtaining the bit weight of the sub-period data; generating a sub-period data according to the brightness data and the bit weight of the sub-period data; carrying out the display of the sub-period data sub-period by sub-period, enabling the accumulated effective lighting time of an LED lamp in the light modulation period to be corresponding to the value of the brightness data, enabling at least some data bits of the brightness data to jointly serve as remainder bits, and specifying the bit weights of the remainder bits. The method and device can reduce the calculation burden of the device while increasing the brightness level.