A method for oscilloscope 3D mapping in scan mode. The input signal is acquired using a real-time sampling rate which is Dr times higher, thus more sampling points, i.e. Dr acquired data can be obtained during the time interval between two consecutive horizontal pixels. The Dr acquired data are mapped into a same column of the screen to implement fluorescent waveform display. In addition, to realize the scanning display, a flag X is introduced into the three-dimensional database, when the screen refresh signal arrives, the first Ds acquired data are read out from the unread acquired data in FIFO memory. The three-dimensional database is updated from the flag X, which make the leftmost waveform always be the oldest waveform, the rightmost waveform always be the newest waveform. Thus the 3D mapping is realized in scan mode, letting the DSO have a fluorescent waveform display at slow time-base.