The disturbance of the surrounding soil by tunnel excavation will inevitably lead to surface settlement. When calculating the surface settlement caused by tunnel excavation, various prediction methods have assumed that the tunnel convergence mode is bilaterally symmetrical. This assumption ignores the influence of asymmetric convergence of the tunnel, and the resulting surface settlement is also symmetrically distributed. To address this limitation, this study proposes an asymmetric tunnel convergence mode and develops a corresponding surface settlement prediction model. The proposed convergence mode decomposes tunnel deformation into three components: uniform radial convergence induced by ground loss, biased ovalization deformation under asymmetric pressure, and rigid-body translation along the bias direction. Three bias-related parameters are introduced to quantitatively characterize the asymmetric convergence behavior. Based on the stochastic medium theory (SMT), a prediction model for surface settlement caused by the bias tunnel was obtained using coordinate transformation and double integral numerical processing. Through actual engineering cases, the applicability of this method was verified, and the influence of relevant parameters on surface settlement was analyzed. The proposed model extends conventional SMT from symmetric to asymmetric tunnel convergence and provides a practical analytical tool for predicting surface settlement in biased-pressure tunnelling conditions.