Since the conduction activation energy of a proton is low, proton-conducting solid oxide fuel cells (H-SOFCs) can work at low temperatures (around 600 °C), which is conductive to maintaining the long-term stability of SOFCs. Proton conductor BaCe_1-xY_xO_3-δ is the most commonly used electrolyte material for H-SOFCs, which directly affects the performance of SOFCs. For the purpose of realizing the high-performance BaCe_0.8Y_0.2O_3-δ (BCY) proton electrolyte materials for SOFC, the effect of different contents of the transition metal oxide NiO on the performance of proton electrolyte BCY was studied. Specifically, BCY-x mol% NiO (named BCY, BCY + 0.5NiO, BCY + 1NiO, BCY + 2NiO, and BCY + 3NiO, when x = 0, 0.5, 1, 2, and 3, respectively) composite electrolyte materials were prepared, and their crystal structures, morphologies, and electrochemical properties were explored. The results showed that NiO could effectively improve the density of BCY prepared at 1400 °C and enhance the proton conductivity of BCY and the performance of BCY-based SOFCs. Among them, the BCY + 2NiO electrolyte had good density and the highest proton conductivity, and the SOFC based on the BCY + 2NiO electrolyte had the best electrochemical performance, indicating that the optimal content of the NiO additive was 2 mol% in this case. The results reported in this work are conductive to the realization of high-performance proton electrolyte materials for the SOFC.