The existence of seed electrons is the precondition of air breakdown induced by high power microwave (HPM). Seed electrons are usually assumed to exist in background atmosphere when simulating the air breakdown triggered by HPM. However, this assumption may lead to some large errors especially in lower atmosphere where the number of electrons is very small. We establish a physical model of seed electron production from O- detachment collision with air molecules using the Monte Carlo method. A three-dimensional Monte Carlo program is developed to simulate this process. The average energies of O- and the average generation time of seed electrons under different electric intensities, frequencies, air pressures and breakdown volumes are obtained through simulation. The simulations show that the average generation time of seed electrons becomes longer with the increase of air pressure or the HPM frequency. The average seed electron generation time becomes shorter with the increase of electric intensity or breakdown volume. Finally, we simulate the processes of O- detachment collision with air molecules under the same experimental conditions. The comparative results show that the seed electron generation from O- detachment can explain the experimental results when the HPM frequency is low, while at higher frequencies, the average seed electron generation time becomes so long that it cannot correspond to the experimental value. Therefore some other mechanisms should be considered in the higher frequency case.