Abstract: A plastic tunnel with external thermal insulation has been a new type of greenhouse to combine the traditional solar greenhouse and plastic tunnel in recent years. The thermal insulation plastic tunnel presents the cost-saving construction and a higher land utilization rate, compared with the solar greenhouse, while a larger working space and excellent thermal insulation performance, compared with the plastic greenhouse. However, the thermal insulation of the enclosure structure can be relatively weak without heat storage and release, compared with the solar greenhouse. The soil can serve as only one regenerator and heat source of the tunnel during heat transfer. Therefore, the soil temperature can determine the heat storage and release, further to form the thermal environment of the tunnel. This study aims to quantitatively analyze the temperature distribution and the heat storage-release property of soil in a plastic tunnel with external thermal insulation. A numerical simulation was conducted to clarify the formation, variation, and influencing factors of the thermal environment in the thermal insulation plastic greenhouse. An experimental test was also carried out in the production greenhouse in the severely cold regions. The results show that: 1) The amplitude of soil temperature decreased significantly, with the increase of depth in a power function. The average thickness of the heat storage layer in the tunnel soil in the test area was determined as about 0.55-0.80 m, according to the Temperature Difference Method. 2) There was a significantly different variation in the lateral soil temperature under the various solar radiation, crop cultivation, and soil marginal effect, particularly in the depth of 0-0.15 m. The soil temperature of each layer in the tunnel was higher than that outside the tunnel all day, and the heat transfer in the transverse soil was in a state of heat loss all day. The loss of heat transfer in the transverse soil was accounted for 9.8%-24.7% of the total heat loss of the soil. Once the heat loss was reused to convert into the soil temperature, the average soil temperature in the tunnel increased by 0.3-0.5 °C. (3) There was no influence of weather conditions on the heat flow in soil, but on the heat storage and release performance of soil. The accumulated heat on the sunny days was about 37.2%-50.6% more than that on the cloudy days, and the release was 44.7%-64.3% more than that on the cloudy days. The maximum heat storage flow and accumulated heat in the sunny days were more than 4 times those on the cloudy days. There was little difference in the soil heat release between the sunny and cloudy days, compared with heat storage. 4) The heat storage and release of soil were depended mainly on the temperature difference between surface soil and air temperature, followed by the solar radiation outdoor, but the smallest impact was the temperature difference inside and outside the tunnel, indicating a convective heat transfer for the heat storage and release of soil in the tunnel. The soil temperature of the tunnel was measured in the middle latitude cold areas. As such, a more accurate heat-transfer model of soil was achieved to clarify the boundary conditions and heat transfer in the tunnel. The finding can also provide theoretical support to promote the plastic tunnel with external thermal insulation in the alpine and high altitude areas.