The present invention provides a frequency conversion measurement circuit for over-excitation protection and an over-excitation protection method. The circuit can measure the multiples of over-excitation in real time, has high conversion speed and high precision, and is just slightly affected by frequency with appropriate circuit parameters provided. Since judgment on a zero crossing point is not required, the inherent delay of the protection is short, and therefore, requirements of the over-excitation protection for action time and return time can be satisfied; and a temperature compensation circuit is adopted, so that accuracy error caused by temperature variation can be effectively prevented. According to the over-excitation protection method, the frequency conversion measurement circuit is adopted to convert signals to be tested into direct current voltage signals, and the direct current voltage signals are outputted; modes for judging whether the protection acts are flexible, hardware can be used to directly judge whether the protection acts, and software can be also used to indirectly judge whether the protection acts; a plurality of parabolas are adopted to fit an inverse-time curve, so that the problem of unsmooth time variation on fitting points can be solved; and a heat effect accumulation discrimination algorithm is adopted, so that the cumulative effect of historical values in an over-excitation value dynamic variation process can be reflected.