N131 is a typical infrared dust bubble showing an expanding ringlike shell. We study what kinds of CO line ratios can be used to trace the interaction in the expanding bubble. We carry out new CO (3-2) observations towards bubble N131 using JCMT 15-m telescope, and derive line ratios by combining with our previous CO (2-1) and CO (1-0) data from IRAM 30-m observations. To trace the interaction between the molecular gas and the ionized gas in the HII region, we use RADEX to model the dependence of CO line ratios on kinetic temperature and H_2_ volume density, and examine the abnormal line ratios based on other simulations. We present CO (3-2), CO (2-1), and CO (1-0) integrated intensity maps convolved to the same angular resolution (22.5"). The three different CO transition maps show apparently similar morphology. The line ratios of W_CO_(3-2)/W_CO_(2-1) mostly range from 0.2 to 1.2 with a median of 0.54+/-0.12, while the line ratios of W_CO_(2-1)/W_CO_(1-0) range from 0.5 to 1.6 with a median of 0.84+/-0.15. The high CO line ratios W_CO_(3-2)/W_CO_(2-1)>0.8 and W_CO_(2-1)/W_CO_(1-0)>1.2 are beyond the threshold predicted by numerical simulations based on the assumed density-temperature structure for the inner rims of ringlike shell, where are the compressed areas in bubble N131. These high CO integrated intensity ratios, such as W_CO_(3-2)/W_CO_(2-1)>0.8 and W_CO_(2-1)/W_CO_(1-0)>1.2, can be used as a tracer of gas compressed regions with a relatively high temperature and density. This further suggests that the non-Gaussian part of the line-ratio distribution can be used to trace the interaction between the molecular gas and the hot gas in the bubble.