This paper introduces a new design method for a high-power density GaN MMIC amplifier operating in the Ku-band. A thermal model to investigate the thermal distribution of power amplifiers is proposed to achieve optimal performance in terms of power density, chip size and channel temperature. The thermal distribution of a single device, an eight-way parallel device combination, and the entire PA layout are obtained by finite element simulation. The channel temperature characteristics of the single device, eight-way parallel device combination, and the entire PA are compared under pulse excitation. The thermal coupling effects of high-power MMICs are analyzed in detail. The thermal resistances are extracted from the simulation to design a Ku-band amplifier. Measurement results demonstrate that the designed amplifier achieves 43.0-44.2 dBm output power and 22.7-34.5% PAE at 28 V drain voltage with a 100 µs pulse width and 10% duty cycle within 12-18 GHz. The proposed design method enables the amplifier to have a compact layout of 10.88 mm ² and a power density between 1.84 W/mm and 2.42 W/mm. This design method can offer valuable insights for future development of high-power MMIC amplifiers.