The objective of this research was to measure firmness of‘Cuiguan’ pears on-line by using visible/near infrared spectrum, also the effect of different pretreatment methods and different points of smoothness on calibration performance were studied. The spectrometer of USB4000 was used to acquire the spectra of ‘Cuiguan’ pears at fruit moving speed of 0.5m/s, and partial least squares (PLS) was used to develop calibration. The total number of samples used was 176, and 119 samples were used as calibration set, 57 samples were used as validation set. The results indicated that it was feasible to measure firmness of ‘Cuiguan’ pears by using visible/near infrared spectrum, and wavelength range of 580~840nm was suitable for calibration. The best calibration performance was obtained when using spectra pretreated by first derivative combined with SNV, the correlation coefficient, root mean square error of calibration (RMSEC) and root mean square error of validation (RMSEP) were 0.820, 2.50N and 3.02N, respectively. For Savitzky-Golay method, the five data points obtained the best calibration, the correlation coefficient, RMSEC and RMSEP were 0.848, 2.31N and 2.85N, respectively.