Abstract: To establish a scientific and quantitative evaluation system for the scale of underground power station buildings, with the aim of enhancing the safety and economy of engineering design.The concept of compactness index based on the equivalent volume ratio of a single turbine unit to the excavation volume of the main power house was proposed.Through statistical analysis of data from 20 typical underground powerhouse projects of hydropower stations such as Three Gorges and Baihetan, and combined with the definition of the distribution interval of the normal distribution function, a quantitative evaluation standard for power house scale covering three levels: loose (scale of main plant building <2.73%), moderate (2.73% ≤ scale of main plant building ≤4.57%), and compact (scale of main plant building >4.57%) was established.Taking the main powerhouse of the right bank underground power station of the Wudongde Hydropower Station as an example for quantitative evaluation.The results showed that its compactness index was 5.39%, which was in the compact range.The scale of the main power house was a compact power house, and the evaluation results were in good agreement with the actual design on site.The compactness index could effectively quantify the size of the water turbine unit occupying the space of the main power plant, and the established grading standards could quantitatively evaluate the scale of the underground power plant main power plant, providing an optimization directions for the safe and economic layout of the main power plant.