To address the problems of difficult feature extraction and low classification accuracy in soft fault diagnosis of DC-DC circuits, a fault diagnosis method based on multi-strategy improved Harris optimization algorithm-back propagation (MHHO-BP) neural network is proposed. The method processes the fault signal by VMD, extracts its time-domain and frequency-domain features as the fault vector, and uses the MHHO algorithm to optimize the weights and thresholds of the BP neural network to establish a VMD-MHHO-BP soft fault diagnosis model of DC-DC circuits. The experimental results show that for soft faults of DC-DC circuits, the method has good diagnostic effect and high accuracy compared with the Whale Optimization Algorithm (WOA) and Butterfly Optimization Algorithm (BOA) optimized BP neural network.