Traditional chaotic detection methods have many problems, such as low criterion accuracy and delay state response. To cope with these problems, a weak signal detection method based on dominative frequency power ratio derived from system’s first-order perturbation solution is proposed in this paper. This algorithm is ascribable to the all-around analyses of chaotic state’s global property and system solution’s frequency-domain characteristics. It not only gives an effective and accurate critical threshold which could offer more reliable guarantee for signal detection, but also disclosures the differences between system states and the coherent physical meanings. The first-order perturbation equilibrium solution of Duffing-Van der pol oscillator is derived with parameter perturbation method, and it is proved that this solutionis is most significant to the dominative frequency. And then, the effective signal is selectively reconstructed through empirical mode decomposition, and system state is redefined with this ratio restrained under MMSE criterion. Finally the mapping relationship between power ratio of dominative frequencies and driving motivation amplitude is obtained and it is considered as determination criterion of critical threshold. Experimental results show that this algorithm could bring an promotion about one order of magnitude in system reliability, and the response speed is at least doubled compared with traditional methods.