The probabilistic analysis of face stability of shield tunnel at present mostly focuses on the variability of soil parameters. However, few researches consider the variability of support pressure. In this paper, based on the K-L expansion, three dimensional random fields of soil parameters are generated to analyze the impact of variability of soil parameters on the failure mechanism of the tunnel face and the critical support pressure.Otherwise, the influence of mean value and the coefficient of variation of the support pressure on the failure probability has been discussed for determining the characteristic mean value of the critical support pressure. The results show that the spatial variability of cohesion and internal friction angle has an important influence on the tunnel face stability, and the influence of variability of internal friction angle is more significant.The failure mechanism of the tunnel face is closely related to the local distribution of random field. When the local random field of the tunnel face is dispersed, the local failure may occurs. The failure probability of the tunnel face is relevant with the mean value and the coefficient of variation of the support pressure. The failure probability decreases with the mean value of support pressure increase or the coefficient of variation of support pressure decrease. The concept of characteristic mean value of the critical support pressure has been proposed, and the determination procedure is given in combination with the failure probability.