The objective was to improve the estimation accuracy of the Blaney-Criddle (BC) method to estimate potential evapotranspiration (ETp) in Guanzhong region of Shaanxi Province, and search for some theoretical basis for the choice of formulas. The Penman (PE) method is widely used all over the world to estimate ETp since it can provide the satisfactory estimations, but it also requires a lot of climatic variables and complicated nonlinear processes in computing ETp. Developing a simple and appropriate method with limited data is urgent. Accordingly, the Blaney-Criddle method was modified. The calibration process used the daily climatic data from 1960 to 1999 and the verification process used the daily climatic data from 2000 to 2015. The results showed that the BC method underestimated the values of monthly ETp when the temperature was low and the BC method overestimated the values of monthly ETp when the temperature was high. By modifying the original BC method, the slope of regression curve of ETp values between the PE method and modified BC method became 0.9997 from 0.685. Moreover, the trend of ETp estimated by the modified BC method was more similar to that estimated by PE method. Compared with the PE method estimation results, the relative error of the ETp values estimated by modified BC method became 1.290%~3.630% from -18.022%~16.269%; the root mean square error of the ETp values decreased from 0.529~0.921mm/d to 0.214~0.283mm/d; the average deviation value became -0.001121~0.000737mm/d from -0.063~0.601mm/d; and the fitting decision coefficient increased from 0.942 to 0.966. It was found that the values of ETp estimated by the modified BC method agreed better with that estimated by the PE method by comparing the year and monthly ETp values. Therefore, the modified BC method can significantly improve the estimation accuracy of the ETp values and it is applicable in Guanzhong region of Shaanxi Province in China.