Reducing the space charge effect and the time dispersion caused by the edge field effect of the scanning deflection system is the key to realize the 100-femtosecond streak tube. In this paper, a novel femtosecond streak tube is proposed and designed. The factors affecting the temporal resolution are analyzed theoretically and the specifications are given. Parameters including the electric field distribution and electron transmittance on the two common acceleration system structures (planar cathode -mesh accelerating electrode and planar cathode – slit accelerating electrode) are compared and analyzed theoretically. The results show that although the electric field distribution formed by the planar cathode – mesh accelerating electrode could form uniform electric field, the electron transmittance is very low; planar cathode-slit accelerating structure would defocus the photoelectron beam along the scanning direction, but the electron transmittance in the effective detection range of the cathode is as high as 100%. The defocusing of the photoelectron beam can be removed by setting a narrow slit in front of the anode. The focusing electrode adopts two groups of plate-like structures which are vertically placed front and back, forming one-dimensional focusing electric fields along the scanning and the slit direction, respectively. The spatial focusing electrode is placed close to the phosphor screen, which is beneficial to push back the cross-point of the electron beam along the spatial direction. Thus, the electron transit time dispersion in the condition of large electron density would decrease. At the same time, the anode can provide a post-accelerating voltage of +5000 V, which is beneficial to shorten the transit time and dispersion of the photoelectrons, thereby improving the temporal resolution. Based on the above theoretical analysis, a novel femtosecond streak tube is designed by using planar cathode-slit accelerating electrode, anisotropic focusing system and post-accelerating method. The influence of the anode slit width on the spatial and temporal resolution is simulated. The results show that the temporal resolution deteriorates with the increase of the anode slot width (10 μm ~ 50 μm), due to the increase of the anode slit width will lead to the gradual increase of the size of the electron spot along the scanning direction, which would lead to the increase of the technical time dispersion. In addition, this study gives the simulation results of the femtosecond streak tube when the anode slit width is in the range of 10~50 μm. The results show that the static spatial resolution is higher than 100 lp/mm @ MTF=10%, dynamic spatial resolution is higher than 29 lp/mm @ MTF = 10%, the temporal resolution is better than 122 fs in the range of 4 mm cathode effective detection length. When the effective detection length of the cathode is increased to 8 mm, the dynamic spatial resolution of the streak tube tube is higher than 22 lp/mm @ MTF=10%, and the temporal resolution is better than 191 fs.