X-ray streak camera is requested to obtain as much information as possible in single measurement because of the picoseconds duration in plasmas diagnosis. An effective method is to enlarge the imaging area. An X-ray streak tube is designed based on the electrostatic focusing system of bi-electrode concentric sphere to solve the problem of imaging quality for large format. Through simulation of the electrons emission by Monte Carol methods and tracing the electron trajectories by Runge-Kutta, the calculation shows that the effective working area of photocathode is 50 mm, central static limit spatial resolution is 50 lp/mm, edge static limit spatial resolution is 31 lp/mm, the magnification is 1.1, the length is 600 mm, and the diameter is 100 mm. The results show that it can satisfy the requirement of large format imaging and can be applied in the process of ultrafast event to obtain lots of information.