Abstract: To accurately evaluate the uncertainty in the calibration factor of a β-ray dosemeter, a traceable thin window ionization chamber was used to measure the reference dose rate at the reference position of the β-ray radiation field, and then the β-ray dosemeter was placed in the same position for measurement to obtain the indicated value. The calibration factor of the β-ray dosemeter was obtained, and the uncertainty in the calibration factor was evaluated. The main sources of uncertainty include the uniformity of the reference radiation field, the uniformity of the position in the measurement process of the substitution method, and the statistical standard deviation of the indicated value. The results show that in the calibration process, the uncertainty in the calibration factor can be reduced by improving the uniformity of the reference radiation field and the uniformity of the reference position.