The suppression of altitude channel error of single-axis rotary Inertial Navigation System (INS) is studied. At first, analysis is made to the stability of altitude channel in pure INS, and an error model is established by using specific force equation. Then, the altitude channel error equation under the condition of single-axis rotary INS is established, and the conditions for suppressing altitude channel error divergence are analyzed. It is concluded that the altitude channel error can be suppressed efficiently when the rotation axis is in horizontal state. Finally, simulation is made to the altitude channel error of INS, influenced by single-axis continuous rotation under the condition that there is bias in triaxial accelerometer. Theoretical analysis and the simulation results show that the altitude channel error can be restrained effectively when the rotation axis of inertial measurement unit is in horizontal state.