Emergency medical services (EMS) are essential components for post-disaster rescue activities. Considering injury heterogeneity and deterioration over time of the casualties, the uncertainty in the number of the casualties can effectively improve the medical service performance. This paper develops a robust optimization model for combined facility location and casualty transportation under uncertainty in the number of casualties. We divide casualties into two types: mild casualties who are transported to on-site clinics with rescue vehicles and serious casualties who are transported to general hospitals with helicopters. Meanwhile, we consider the Injury Severity Score (ISS) increment to describe the injury deterioration of the casualties over time. The objective is to minimize the total weighted ISS increment of mild casualties and serious casualties. Then, we employ the robust optimization method to deal with the uncertainty and derive the robust counterpart of the proposed model in this paper. Case studies based on Yushu Earthquake show that the model can get the optimal emergency facility location and casualty transportation scheme to minimize the total weighted ISS increment. The total weighted ISS increment increases as the constraint violation probability decreases, which reflects the trade-off between performance and robustness. Sensitivity analyses show that the greater uncertainty of the casualty number is, the greater the impact on the total weighted ISS increment is, the more conservative the decision scheme is. The capacity of general hospitals has a greater effect on the objective value compared with the capacity of on-site clinics and the decision scheme of the robust optimization model has a greater optimality compared with the deterministic model when the problem size is magnified.