State of charge (SOC) estimation of lithium-ion battery is of vital significance for the control strategy in the battery management system (BMS). To integrate the spatial and temporal characteristics of the data, and to accomplish the transfer of knowledge, a novel convolutional neural network-bidirectional long short-term memory network based on transfer learning optimization (CNN-BiLSTM-TF) is proposed to estimate SOC. In detail, the spatial and temporal features hidden in the data are learned through CNN and BiLSTM, respectively. Furthermore, the CNN-BiLSTM network is established under one working condition and transferred to other working conditions through transfer learning, from which the SOC can be estimated online. A lithium-ion battery dataset is applied to verify the proposed SOC estimation approach. The results of case study demonstrate that the estimation performance of CNN-BiLSTM-TF is better than some existing approaches.