An ultra-sensitive nanoscale magnetism sensing of van der Waals layered CrSBr is demonstrated by employing optical whispering-gallery-mode (WGM) resonances in a self-rolled-up microcavity. CrSBr nanoflakes with and without intercalation are probed by monitoring the WGM resonant modes shift as a function of magnetic field intensity and temperature, revealing the presence of ferromagnetic (FM) and antiferromagnetic (AFM) order. The minimum detectable amount of CrSBr nanoflakes reaches the femtogram scale, and the detection limit is as low as 1.25 × 10⁻¹⁶ emu. This study demonstrates a promising application of WGM microcavities for highly sensitive magnetism detection with reduced system complexity, establishing a novel bridge between materials’ optical and magnetic properties.