The experiment of diode side-pumped Nd∶YLF multi-pass amplification was studied. The high gain amplifying cavity of diode side-pumped Nd∶YLF was designed optimally, to avoid the self-emission of amplifier. In the amplifier laser wavelength was 1053 nm, laser material was c-axis Nd∶YLF, repetition rate was 1 Hz, four-pass structure was used, the total pumped power of amplifier was 1.8 kW, the central wavelength of diode was 797 nm, in the amplified cavity the diode was arranged compactly, and the high pumped efficiency was obtained. In the amplifier the seeder was 0.1 μJ, 6.5 ns, M2≤1.1 and stability was ±8%, the output was 2.9 mJ, 5.2 ns, mean of M2 was 1.65, stability was ±6.9%, total gain was 2.9×104.