An adjustment method of an atmos clock and a detection system performing the adjustment method are provided. The adjustment method includes detecting a magnetic field around a balance wheel of the atmos clock via a magnetic sensor, generating a measured signal, receiving the measured signal via a processing unit, and performing a clamping process on the measured signal. A clamping value is subtracted from the measured signal to generate a clamped signal. A zero value line crosses a waveform of the clamped signal to form a zero crossing point in each of the periods of swing motion of the balance wheel. The adjustment method further includes retrieving a time of the zero crossing point, calculating a time difference between the zero crossing points of two adjacent periods, calculating a length of the period according to the calculated time difference, and outputting the calculated length to an output device.