We report an ultrasensitive atomic spin sensor employing double spin resonance, achieving a spin signal enhancement of 2600 and a sensitivity of $0.57\mathrm{fT}/\sqrt{\mathrm{Hz}}$ under nonzero magnetic field measurements. Furthermore, we establish an in situ alkali-noble-gas spin sensor by tracing the measured spin precession frequency to the nuclear spins gyromagnetic ratio constant with high accuracy. The dominant systematic uncertainty induced by Fermi-contact interactions during the trace process has been suppressed by more than 2 orders of magnitude via pulsed train sequences. This Letter paves the way for achieving both ultrahigh sensitivity and traceability, offering promising opportunities for exploring new physics beyond the standard model and advancing quantum metrology in complex environments.

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• Atomic & molecular collisions
• Coherent control
• Dark matter
• Light-matter interaction
• Optical pumping
• Quantum measurements
• Atomic ensemble
• Atoms