In order to explore the formation causes and rules of Poisson's burr in ultra precision cutting of nickel-phosphorus alloy， and to seek a more accurate characterization method of Poisson's burr size， this paper conducted theoretical analysis of the formation mechanism of Poisson's burr in nickel-phosphorus alloy and established a prediction model for the height and width of Poisson's burr in orthogonal cutting process. Through experimental analysis of the influence of cutting parameters on the height and width of Poisson’s burr， it can be observed that the effect of cutting depth on burr size is significant. When the cutting depth increases from 3 $\mathrm{\mu }\mathrm{m}$ to 9 $\mathrm{\mu }\mathrm{m}$， the burr height increases by 0.099 8 $\mathrm{\mu }\mathrm{m}$ and the burr width increases by 1.06 $\mathrm{\mu }\mathrm{m}$， while the cutting speed has little effect on the burr height and width. By comparing the burr data obtained from the experiment with the predicted data， it can be seen that the average relative error values of the burr height and width prediction models are 5.43% and 8.17%， which verifies the accuracy of the prediction model. At the same time， the current characterization method of the height and width of Poisson's burr have certain errors. Therefore， this paper proposed a method for calculating the volume of Poisson's burr using the integration method， and established a more accurate characterization method of the size of Poisson's burr based on the volume method. The average relative error value of the predicted volume model is 4.81%， indicating that the accuracy of the Poisson's burr volume prediction model is relatively higher. The research results provide theoretical guidance for the rational selection of cutting parameters and Poisson's burr evaluation method in the orthogonal cutting process of nickel-phosphorus alloy.