During the defrosting operation of air source heat pump (ASHP), it stops supplying heating, or even absorbs heat from indoors. This results in the significant fluctuation of indoor thermal environment. However, the variation rule of indoor thermal environment during frosting-defrosting is still unclear, which severely restricts the heating quality improvement of ASHPs. To solve this problem, two test rigs, an air-water heat pump (AWHP) heating system and an air-air heat pump (AAHP) heating system, were built in an artificial environmental chamber. Through creating the frosting environmental condition, the practical space heating experiments of ASHPs during frosting-defrosting process were conducted. Then, the indoor thermal environment variation rules of the two ASHPs and their differences were analyzed. Results show that the indoor temperature during the defrosting process of both the AWHP and AAHP decreases obviously, and the decline amplitude in rooms using AWHP is greater. The temperature decline in rooms using AWHP is 2.99–4.66 °C, while 1.58–1.79 °C in the room using AAHP. For AAHP, during the later stage of its defrosting operation, the supply water temperature drops below the indoor air temperature, causing the indoor fan coil units to absorb heat from the indoor air instead of providing heating.