The invention discloses a device and a method for testing DCS (distributed control system) response time. The device comprises a DCS signal input unit, an oscilloscope, a control system and a display of the control system, wherein the oscilloscope is connected with the input unit, the control system and the display thereof are respectively used for controlling and displaying signals to be tested,and the oscilloscope is connected with a photomultiplier tube capturing variation of pictures of the display. Variation of dynamic icons and characters on the display are scanned according to the characteristic that the photomultiplier tube is sensitive to color changes, so that accurate time, for displaying status change of the input signals, of the control system is obtained. Received photoelectron is multiplied by secondary electron emission of the photomultiplier tube, so that slightly varied optical signals can be tested. The photomultiplier tube can output different electric signals to light with different wavelength, and accordingly, color change on the display is converted into voltage change. Relative response time can be tested directly according to the time difference obtained by comparing change of the input signals on the oscilloscope or waveform recorder to change of output signals of the photomultiplier tube.