A high-birefringence spiral Sagnac waveguide (SSW) device fabricated via direct laser writing (DLW) using a two-photon polymerization (2PP) technique is proposed, designed, and experimentally demonstrated as an ultrahigh magnetic field sensor. The sensor comprises a Y-style tapered waveguide and an SSW containing two microfluidic channels. The SSW has a total length of $\sim 2.4\mathrm{mm}$ and a spiral radius of $\sim 200\mathrm{\mu m}$. Due to the asymmetric structure, the SSW has a high birefringence of 0.016, which can be designed as a magnetic field sensor, as a magnetic fluid can be filled into the microfluidic channel changing the guiding mode and the birefringence and consequently leading to a change in phase of the interferometer when the applied magnetic field changes. The experimental results show that the proposed photonic device has a sensitivity to magnetic fields as high as 0.48 nm/Oe within a range from 10 to 100 Oe. The proposed device is very stable and easy to fabricate, and it can therefore be used for weak magnetic field detection.