In shield tunneling, the excavation stability is very important. The balance pressure which supports the work face is offered by the the mucks in the earth chamber. The ideal supporting pressure on the excavation face is trapezoidal. In fact, the pressure is irregular. So, the conceptions of earth pressure supporting ratio (EPSR), regular modulus of earth pressure and buffer ability of chamber are put forward. Earth pressures in two different situations of shield tunneling in clay ground and cobble sand ground are studied. The studies show that EPSR and regular modulus of earth pressure in soft ground are both better than those in cobble ground. EPSR and regular modulus of earth pressure are two appropriate assessing indexes for EPB tunneling.