The mode number of optoelectronic oscillators is the key parameter to establish the relationship between the oscillation frequency and loop group delay in the distance measurement method based on optoelectronic oscillators (OEO), and the mode number error can affect the accuracy directly. Due to the low frequency stability of OEO, the accuracy of mode number by measuring the oscillation frequency and inter-mode frequency spacing is not high. The influence of frequency stability on the mode number error and the change law of frequency drifts are analyzed. Then a new method to determine the mode number without changing the structure of OEO is proposed. In this method, the frequency spacing between oscillation mode and x-order-higher mode and the frequency spacing between oscillation mode and x-order-lower mode are measured separately to get the inter-mode frequency spacing, in order to compensate and reduce error induced by frequency drifts. The mode number measurement error model of this method is established. Experiments are conducted by using different methods, and the experimental results are consistent with error analysis.