The signal characteristic of a magnetic object, such as a submarine, is indeterminate in the magnetic object detection, which may appear as a magnetic anomaly signal or an extremely low frequency magnetic signal with undetermined coefficients. This paper proposes a universal fast detection method for both the weak magnetic anomaly signal and the weak extremely low frequency magnetic signal with undetermined coefficients. A sineGauss mixture model is built based on the piecewise sinusoidal statistical characteristics of the magnetic anomaly signal and extremely low frequency magnetic signal. This universal model can represent both the magnetic anomaly signal and extremely low frequency magnetic signal with undetermined coefficients. A detector is developed based on the sineGauss mixture model and the sequential detection theory, which realizes the universal fast detection for both types of signals mentioned above. The unknown parameters in the detector are identified, and the sequential detection performance of the detector is studied. In addition, the detection performance for different weak magnetic anomaly signals and weak extremely low frequency magnetic signals is analyzed. The universality and rapidity of the designed detector for both the weak magnetic anomaly signal and the weak extremely low frequency magnetic signal are verified based on experiment system with geomagnetism. The experimental results indicate that the signaltonoise rate can be -8 dB, the amount of calculation is reduced by 4 orders compared with the traditional detection methods.