1. A down sampling method comprising: extracting energy statistical values of sampling point sets in a current period; selecting a sampling position corresponding to a sampling point set with the largest energy statistical value as a down sampling position; and performing down sampling according to the down sampling position, wherein extracting the energy statistical values comprises: taking statistics of energy values of sampling points in the sampling point sets; outputting energy statistical values of the sampling point sets in one burst impulse sequence; calculating adjustment factors according to the energy statistical values of the sampling point sets in the one burst impulse sequence; multiplying the energy statistical values by the adjustment factors; and accumulating the products of the energy statistical values and the adjustment factors in one period to obtain the energy statistical values in the current period.