A method for adjusting the waveform brightness for a waveform formatted to be displayed on a digital three-dimensional (3D) oscilloscope having M brightness gradation levels to display the waveform on a digital 3D oscilloscope having L brightness gradation levels is includes, creating a ROM in an FPGA and storing a look-up table of screen display brightness value of LCD that is corresponding to the waveform occurrence N(T,A) at the current brightness gradation L. The ROM is divided into 2a sub ROMs, each sub ROM has the capacity of 2b×d bits. A value of round(pL·N(T,A) is assigned to waveform brightness value D(T,A) and is stored correspondingly into the subROML of 2b×d bits by ascending order of the b bits of binary data of waveform occurrence N(T,A). In this way, using the b bits of binary data of waveform occurrence N(T,A) as the binary address of the subROML, corresponding waveform brightness value D(T,A) at the current brightness gradation L can be obtained through look-up table in the subROML.