This article has introduces the basic components and characteristics of a femoto-second electron diffraction system, i.e. the photo-cathode, magnetic lens, deflector and detector with doubled MCP etc., and demonstrated our design method and results. The photo-cathode is an Ag film deposition on a sapphire disk and the femtosecond electron pulses are accelerated to 50 keV through an extraction electric field of 10kV/mm between the photocathode and the extraction mesh. After extraction, the electron pulses are reshaped in size by a 100 μm pinhole, which screens out electrons with larger elevation angle and makes the passed electrons flying almost parallel to the axis, which results in a reduced electron beam spot. The beam is collimated with a magnetic lens and positioned to the sample by a pair of deflection plates for diffraction measurements. The electron diffraction images are recorded with a two-dimensional imaging system composed of a Chevron-type microchannel plate and a P20 phosphor screen on an optical-fiber faceplate. The initial position, kinetic energy and angle distribution of the photoelectrons is sampled with Monte Carlo method. The electric field is calculated with finite difference formula, and the magnetic flux of the focus region is calculated with finite elements methods, and the electron trajectory is traced with Runge-Kutta method. We analysized the temporal and spatial distribution of 3000 electrons when they reached the prime plane and give out the temporal and spatial modulation transfer function of the femoto-diffraction system.