Lead-free double perovskite CsAgBiBr has gained increasing attention recently. However, the power conversion efficiency (PCE) of CsAgBiBr perovskite solar cells (PSCs) is still low compared with their lead-based counterparts. Here, by using photoluminescence (PL), time-resolved photoluminescence (TRPL), and ultrafast transient absorption (TA) measurements, the unbalance between the electron and hole in diffusion and transfer, which limits the performance of the CsAgBiBr PSCs, was further revealed. Considering this issue, a strategy of using the mesoporous TiO electron transport layer (ETL) to construct a bulk heterojunction in CsAgBiBr PSCs was proposed. Consequently, the PCE had improved by over 24% comparing with that only used compact TiO ETL. Moreover, based on mesoporous TiO, the unencapsulated CsAgBiBr PSCs maintained 90% of their initial performance after approximately 1200 h of storage in a desiccator (humidity ~30%). This work gives further understanding of CsAgBiBr perovskite and demonstrates that a proper design of balancing the electron and hole diffusion can improve device performance.