This paper addresses the issue of environmental risk perception,in a multidimensional framework involving perceived risks in both time and space.A representation of the probability-time&space tradeoff (PTST)is obtained in which the probability-time tradeoff (PTT) and the probability-space tradeoff (PTS) are combined.The probability-space tradeoff is analogous to probability-time tradeoff,due to the fact that probabilistic distance,temporal distance and spatial distance are psychologically equivalent.PTST provides a formal model to accommodate the intuition of temporal and spatial distances being a source of uncertainty in the process of environmental risk perception and provide an explicit formula to transform objective distances into subjective probability.Finally,we present an experiment in a between-subjects design in which we manipulate the temporal and spatial distances of a water pollution risk in the questionnaires.The subjects are asked to give their probability judgments towards the spatial-temporal events and they typically believe that they are less likely to suffer health problems when temporal and spatial distances of pollutants extend longer.The results verify the validation of the proposed PTST model.