To understand the active sites requirements for NOx-SCR by NH3 and for designing better catalysts, four SnO2-based solid solution samples with the SnO2 lattice doped by metal cations possessing redox ability (Ce4+ and Cu2+) or acidity (In3+ and W6+) have been purposely designed. It is revealed that the metal cations have entered into the SnO2 matrix to form a tetragonal rutile solid solution phase. Compared with individual SnO2, all the modified catalysts possess higher surface areas, lower crystallinity, more abundant surface defects, facile oxygen, and acidic sites. Therefore, they display improved reaction performance. Both surface facile oxygen and acidic sites play vital roles, and the balance between them controls the reaction. For the catalysts modified by Ce4+ and Cu2+, a better balance can be generated. Therefore, they display the best performance among all the samples. NOx-SCR by NH3 on the SnO2-based solid solutions follows a Langmuir-Hinshelwood mechanism.