In order to improve the accuracy of annual-ring measuring instrument, a new adaptive low-pass filtering algorithm was proposed to filter the direct current (DC) motor current signal of annual-ring measuring instrument. The filter coefficients of ordinary lowpass filtering algorithm were fixed. When the annualring width was changed, the current fileted by ordinary low-pass filter may not only retain part of the noise signal, but also filter part of the effective annual-ring signal, which reduced the accuracy of the annual-ring measuring instrument. The filter coefficients of the adaptive low-pass filter algorithm can be adjusted automatically with the changes of the input and output values of the filter, which can improve the effect of noise filtering and reduce the loss of effective signals. Totally 60 larch discs, including large, medium and small diameter orders were measured by self-manufactured annual-ring measuring instrument. The original current signals of DC motor were recorded. The adaptive low-pass filtering algorithm and the ordinary lowpass filtering algorithm were used to filter the original current signals of DC motor respectively. According to the filtered current waveform, the disc age was estimated, and the annualring recognition accuracy of the two filtering algorithms was calculated. The average annualring recognition accuracy of adaptive lowpass filter was 89.66%, and that of ordinary lowpass filter was 75.84%. The average annual-ring recognition accuracy of adaptive lowpass filter was 13.82 percentage points higher than that of ordinary low-pass filter. When the average ring width became more narrow, the annual-ring recognition accuracy of the adaptive low-pass filter was decreased by 13.59 percentage points from 95.61% to 82.02%, while that of the ordinary low-pass filter was decreased by 24.07 percentage points from 86.37% to 62.30%. The experiment results showed that the adaptive low-pass filter had high annual-ring recognition accuracy, strong self-adaptive ability and stable annual-ring recognition accuracy.