Objective To determine the responses of radioactivity meter at different positions in the vertical direction of the ionization chamber by measuring different radioactive sources. Methods The radiation of cesium-137, cobalt-60, and americium-241 nuclides was measured at different positions in the vertical direction of the ionization chamber with commonly used clinical radioactivity meters. The measured values were fitted to obtain three trend lines. The maximum deviation of the measured values was estimated according to the trend line, and the deviation was estimated at different syringe needle heights. Results According to the trend lines, the maximum deviation of the radioactivity meter CRC-55tR in measuring cesium-137, cobalt-60, and americium-241 nuclides was 5.15%, 5.98%, and 6.25% respectively. The radioactivity meter RM-905a was used to measure three nuclides at different syringe needle heights, and the maximum deviations were −4.33%, −9.9%, and −12.65%, respectively. Conclusion The three nuclides showed different change patterns in measurement with CRC-55tR and RM-905a but similar change patterns in measurement with the same radioactivity meter. The values measured with the same radioactivity meter showed significant deviations at different positions in the vertical direction of the ionization chamber. It is recommended to make position correction for commonly used nuclides. Reducing measurement error and improving measurement accuracy of nuclides are of great significance for radiation protection.