The finite-difference time-domain method (FDTD) is employed to investigate the near-field distribution of the light spot of aperture pyramid-type optical probe with a metal tip. The factors that have some influence on the electric field distribution such as the length，radius and position departed from aperture of the tip are investigated. The mechanism of electromagnetic wave transmitting from the aperture to metal tip by way of plasmon is discussed. Our calculation results are consistent with the results obtained by Taminiau with different methods. How and why Pohl etc. have obtained images with high resolution in the 20—25 nm range between 1984 and 1986 are analyzed. The imaging mechanism of their experiment system is discussed. Numerical simulation demonstrates that the imaging mechanism of Pohls experiment system may belong to a scattering-type near-field optical microscope (S-SNOM) rather than an aperture scanning near-field optical microscope (A-SNOM). This paper can help us to find optimized tip design in the future.