To explore the influences of altitude and alignment conditions along the gradient zone of the Sichuan-Tibet Plateau on CO₂ emissions of light-duty vehicles on expressways, a portable emission measurement system was employed to conduct round-trip real-vehicle tests on the Wenchuan-Barkam Expressway. The variation patterns of vehicle specific power, CO₂ emission rate, and CO₂ emission factor were analyzed. Moreover, correlation models between vehicle CO₂ emission factor and grade was established. The results indicate that altitude has a significant influence on emissions. Based on the emission influence characteristics, the altitude of the test section can be divided into two ranges: 1 400 ~ 2 250 m and 2 250 ~ 3 240 m. The CO₂ emission rate in the altitude range of 2 250 ~ 3 240 m is approximately 40% lower than that in the range of 1 400 ~ 2 250 m. The mean values of CO₂ emission rates in these two altitude ranges are 1.5 g/s and 2.5 g/s respectively. Within the same altitude range, the mean value of CO₂ emission factors for circular curves with a radius in[1 100 m, 1 400 m] is smaller than that for curves with a radius less than 1 100 m. In the altitude range of 1 400 ~ 2 250 m, the mean value of CO₂ emission factors in different grade intervals initially increases and then decreases with the increase of grade, with a range of 21 g/km. In the altitude range of 2 250 ~ 3 240 m, the mean value of CO₂ emission factors in different grade intervals exhibits an S-shape growth trend as the grade increases, with a range of 53 g/km.