To address the issue of frequency instability caused by power imbalance when a HVDC transmission system enters islanded operation, a cooperative control method for DC islanded systems based on positive interactive energy gradient is proposed. First, a dynamic mathematical model of the DC island system is established, along with a corresponding dynamic energy model. Then, the interaction energy terms between different subsystems are extracted, and the positive interaction energy is quantitatively evaluated as a direct factor that degrades system stability. On this basis, the method coordinates the regulation of the outer-loop active power reference of wind turbines and the current reference of rectifiers to compensate the positive interaction energy of the DC islanded system. This enables emergency compensation for the power difference between the power grid at the sending end and rapid restoration of frequency stability. Finally, a DC islanded system model is built on the RT-LAB platform to verify the proposed method. The experimental results show that the proposed cooperative control method for DC islanded systems can significantly reduce the negative influence of positive interaction energy on system stability and realize rapid and stable recovery of system frequency.