High brightness electron beam is one of the main goals of the research and development effort in RF photo-injectors. Compared with the normally used magnetic chicane, an alternative scheme, commonly known as 'velocity bunching', has been proposed as a tool to compress electron beam pulses in modern high brightness photo-injector sources. This paper presents numerical optimization systematically and the first attempt to demonstrate the velocity bunching scheme on SDUV-FEL linac experimentally. The relationship between the degree of bunching and the off-crest phase of the accelerating structure is explored experimentally. Velocity bunching operating mode illustrates flexible performances with an intrinsic machine jitter, which agrees well with the theoretical prediction.