The district heating area in China is continuously increasing, which brings an increase in district heating load. In order to solve the shortage of heating power and realize the carbon neutral target, this study proposes two retrofit schemes for district heating system by integrating air source heat pump and water source heat pump, respectively. Mathematical models are established to study the performance of the integrated systems and a bilevel optimization model is proposed to optimize them. The results show that the air source heat pump combined district heating system has better performance compared to the conventional system, which reduces 50% energy consumption, 10.8% carbon emissions and achieves better economy. The dynamic coupling property of the district heating network and power grid are also considered, and the results indicate that the introduction of air source heat pump can effectively improve the efficiency and stability of power grid and reduce the seasonal fluctuation. The potential of large-scale application of air source heat pump combined system in Beijing is evaluated. The results reveal that retrofit scheme of integrating air source heat pumps into district heating system can cover 2930 MW heating load and bring 362 million Chinese Yuan profit by reducing 219,000 tons of carbon emissions and 539,000 tons of standard coal consumption in 2025.