Plastic greenhouses, broadly applied in southern China, have insufficient thermal insulation and thermal storage, resulting in a poor thermal environment at night. This paper chose a plastic greenhouse in Chengdu, Sichuan province, to measure the thermal environment variation in winter. An energy balance model of the plastic greenhouse was developed and then verified by the measurements. The influence of the ground thermal storage on the indoor thermal environment was studied. As a good thermal mass, the ground soil could regulate the indoor thermal environment at night, especially by increasing the near-ground temperature. At night, the ground surface had higher temperatures than the indoor air, with a temperature difference of 4-6 ?. Further, phase change materials (PCMs) were proposed to improve the ground thermal storage conversion efficiency. The ground surface temperature of the plastic greenhouse with PCMs was higher than that of the plastic greenhouse without PCMs by 1 ?, and the average ground thermal storage conversion rate could be increased by 7%.