The L-band power amplifier (PA) die has a demand for power increase, while facing the problem of large volume. A 300 W die of the GaN HEMT was developed based on 0.5 μm process. The best input and output impedance points of the model were extracted by loadpull simulation. The L-C network was designed with a high dielectric constant thin-film circuit to raise the input and output impedance of the chip and offset the imaginary part. The four-cell core was combined with the two-stage impedance transformed wideband power divider and combiner circuit by using microstrip circuit. A high integrated and miniaturized PA was achieved by building in the stabilization circuit, the gate and drain bias circuit, and achieving a 50 Ω input and output impedance matching. With a total gate width of 4×40 mm, the output power is from 60 to 61.2 dBm, the efficiency is from 57.9 % to 72 % and the saturation power gain is greater than 14 dB under the conditions of a drain voltage of 50 V, a pulse width of 40 μs and a duty ratio of 4%.