Air source heat pumps are vital for the implementation of coal-to-electricity policies in northern China. However, majority of the low-temperature air source heat pumps operate under a single and fixed mode, which may not be suitable for residential applications in regions with varied climates. Therefore, herein, a multimode single-fluid cascade air source heat pump (MSC-ASHP) is proposed, which includes the following modes: cascade compression heating, single-stage compression heating, and single-stage compression regeneration modes. To investigate variable-speed characteristics and control strategy of the two heating modes, the experiments and simulations are performed at different evaporating temperature and lower-stage compressor speed. The results demonstrate that the heating capacity in two modes can be enhanced effectively by adding the lower-stage compressor speed. The coeffinient of performance (COP) of cascade compression heating mode increases first and subsequently decreases along with the increasing of the lower-stage compressor speed. Under a condensing temperature of 46 °C and evaporating temperature of −35 °C, the COP is higher than 1.8, the discharge temperature is lower than 120 °C, and the compression ratio in each stage does not exceed 5.0, indicating this system functions safely and steadily in cold climates. The control strategies for the compressor speed and mode switch are summarized synthetically based on the simulation and experimental results. New heat pump system prolongs the year-round applications period of the low-temperature air source heat pump.