Environmental variation along altitudinal gradients can promote life-history trait differentiation in ectothermic animals. Life-history theory predicts that increased environmental stress results in a shift in reproductive allocation from offspring quantity to quality and a stronger trade-off between egg size and clutch size. To test this prediction, we investigated patterns of variation in life-history traits (i.e. age, body size, clutch size and egg size) among four populations of Bufo andrewsi from Baoxing County, western China, at different altitudes. We found that body size, age, egg size and total reproductive output, but not clutch size, differed between populations. Clutch size and total reproductive output increased with female size and age. However, egg size decreased with female size and did not change with female age. The egg size and clutch size trade-off was evident for all populations except at lowest altitude, and the strength of trade-off between egg size and clutch size increased with altitude. Our findings suggest that environmental constraints at high altitude select for investment in larger eggs at a cost of offspring number.