To clarify the effect of small magnetic fields on the magnetic properties and microstructure in FeCoSiBPC amorphous alloys, a magnetic field with different directions and various intensities was applied. The results show that the static field in orthogonal directions could all decrease Hc. Small fields like 50 Oe are also effective in reducing the Hc. The microstructure of the samples annealed with and without field were compared to figure out the effect of the magnetic field on the Hc. A change in the crystallization growth between field heat treated samples and non-field heat treated samples is observed where the field initially hinders crystallization to accelerate it at longer times. Small magnetic fields could also affect the proportion of bcc-Fe(Co) phase with different Co coordination number and are effective in aligning the magnetic domain, which could result in lower Hc by sweeping pinning site. The magnetic softening and magnetic hardening mechanism are also discussed.