The ground source heat pump (GSHP) is currently employed to improve the overall energy efficiency of the building sector. Many studies have been directed to optimizing the energy efficiency (GSHP) through its source side. The potential influence of air side of GSHP on the energy efficiency is not fully addressed. Yet, ventilation of the GSHP serves as a hub linking indoor thermal comfort and power consumption together. In this paper, we present a systematic study focusing on the ventilation rate of the GSHP system from the perspective of energy saving and indoor thermal comfort combined. This study constructed the mathematical model of PMV (Predicted Mean Vote) and power connected by ventilation rate through a full-scale GSHP system. The results have indicated that the system COP is significantly influenced by the ventilation rate changing from 4 to 2.1. The non-linear decrease of the coefficient of performance (COP) creates a turning point in the PMV ∼ power demand curve of the GSHP system, where the lowest power demand can be achieved at a 0.33 ventilation rate fraction while PMV level is improved from ‘slightly warm’ to ‘nearly neutral’. Finally, the study constructed different scenarios considering different ventilation methods and different cooling capacities.