Using a microlens array to focus an amplified femtosecond laser pulse inside a bulk PMMA(polymethyl methacrylate), optical breakdown occurs at each focus of the microlenses. By changing the positions of the foci,we realize the three-dimensional optical recording of high contrast bits at a speed of 20Mbit/s (with a 1kHz fs pulse laser ) in PMMA. Accordingly, modulation and readout of binary data have been presented. The diameter, focus length,and lens aberration in the microlenses are key factors which affect the recording speed and optical capacity based on this mechanism. Two kinds of effective ways to increase the capacity are discussed in this paper.