Abstract: In order to achieve accurate measurement of the absorbed dose of beta radiation, the main influence factors of the extrapolation chamber entrance window correction factor k_wi were studied. k_wi at different cavity lengths and different sourcedetection distances was measured by experiments. The results show that the correction factor k_wi with the same source-detection distance decreases with the increase of the cavity length. And k_wi decreases with the increase of the source-detection distance when the cavity length is 3 mm. The calculated k_wi without flattened filter at source-detection distance of 30 cm and cavity length of 3 mm was simulated using Monte Carlo method. The simulation result is 1.084 5, which differs from the experiment result by 0.11%. Therefore, in addition to the correction factor k_wi, corresponding source-detection distance and cavity length should be supplied.