Based on Terzaghi's theory on loosening earth pressure, two failure modes of soil upon shallow EPB shield tunnel caused by synchronous grouting are analyzed, and the theoretical equations of synchronous grouting pressure is built to calculate the maximum and minimum grouting pressure which correspond to the limit states of subsidence and upheaval failure. Combined with a shield tunnel construction practice, the theoretical and measured values are compared. The results show that the theoretical equations are valid and can provide a theoretical approach for the parameters of shield synchronous grouting.