Abstract: Space harmonic magnetron(SHM), whose electrons synchronize with high order space harmonic of non-π mode, could realize the development of magnetron towards millimeter wave and terahertz effectively. Compared with the traditional magnetrons synchronized with the fundamental of π mode, the SHM has a larger resonator size and lower magnetic field. But this kind of magnetron has the disadvantage of lower efficiency. In this paper,firstly, using the single electron model, the electron power formula is obtained by analyzing that the wave interacts with the space harmonic in SHM. The electronic efficiency is mainly affected by the angular rotation velocity Ω_m and the angular component E_φm of the m-th electric field. Then, the reason for the low efficiency of SHM is analyzed from angle velocity and space harmonic field. Finally, based on the theoretical analysis, the experimental study is held to improve the efficiency of the SHM on the coupling impedance, radial electric field, and angular electric field. Using these experiment results, a W-band SHM is manufactured successfully, and its electrons interact with the first backward harmonic of π/2 mode. When the anode voltage is 13.6 kV, the prototype is able to deliver an output power of 11.2 kW and an efficiency of 8.2% with a pulse width of 400 ns and a duty factor of 0.42%.