The Ba5SiO4Cl6: Yb3+, Er3+, Li+ phosphor has been prepared by high temperature solid state reaction, and their upconversion (UC) luminescence properties and mechanisms are investigated. The UC emission bands located at 525 nm (2H11/24I15/2), 548 nm (4S3/24I15/2), and 662 nm (4F9/24I15/2) due to Er3+ are observed under the excitation of 980 nm. UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+ phosphors is increased with increasing Er3+ and Yb3+ concentration due to the energy transfer enhancement of Er3+ and Yb3+. Based on the relations of UC luminescence intensity and excitation light power, the UC luminescence mechanisms are discussed. At a low excited power (below 0.8 W), the two-photon processes are involved in both green and red UC emission mechanisms. When the power exceeds 0.9 W, the green and red UC emission is a four-photon process. One new and interesting UC emission mechanism may occur in the Ba5SiO4Cl6: Yb3+, Er3+ phosphors. Both green and red UC emissions at a higher pumping power are generated by photon avalanche UC process. Influence of Li+ doping on the UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+ phosphors is investigated. Result demonstrates that Li+ ion doping could enhance the UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+, which is attributed to the distortion of the local symmetry around Er3+.