Abstract: Protodyakonov′s pressure arch theory has been a mechanical basis for the design of tunnels and underground works in China, but it has been questioned to some extent by academic and engineering fields. This paper analyzes the real meanings and formation conditions of a pressure arch using the FEM strength reduction method and a model test, researches the relationships among tunnel span, rock mass strength and pressure arch self-stability, and discusses the failure mechanism of tunnels under different buried depths. The results show that: 1) the pressure arch theory is only applicable to rectangular or gentle-vault tunnels with stable surrounding rocks and a shallow buried depth; 2) a pressure arch will not occur in an arched tunnel; 3) a self-stable pressure arch will not occur under very low rock strength and a too large tunnel span; 4) a pressure arch and tunnel failure will not occur in a tunnel with stable surrounding rock; 5) for a tunnel with a great buried depth, the failure first occurs on both sides, so the pressure arch theory is not applicable