Terahertz pulses accelerating and scanning electrons can break the limitation of accelerating electric field between cathodes and grids in traditional streak tubes, thus reduce the time dispersion and enhance temporal resolution of time-scanning detectors. Based on this new technology, this paper designs a ultra-small structured time-resolved detector with no focusing pole. The terahertz pulse coupling/enhancing device suitable for acceleration zone and scanning zone is designed and optimized. The enhancement coefficient of the terahertz pulse electric field in the device reaches 7.03 and 9.39. The paper theoretically analyzes the relationship between time dispersion in acceleration zone and the moment of electrons emission. We also analyze the influence of space charge effect on time dispersion. The electronic trajectory tracking is used to calculate and analyze the time dispersion of this detector, and finally the time resolution is better than 50fs.