The dependency of phosphorous content on the glass forming ability, thermal stability and soft magnetic properties of Fe83.4Si2B14−xPxCu0.5C0.1 (x=0,1,2,3,4) alloys was investigated. The experimental results showed that the substitution of B by P increased the glass forming ability in this alloy system. The Fe83.4Si2B10P4Cu0.5C0.1 alloy shows a fully amorphous character. Thermal stability of melt-spun ribbons increases and temperature interval between the first and second crystallization peaks enlarges with the increase of P content. And the saturation magnetic flux density (Bs) shows a slight increase with the increase of P content. The Fe83.4Si2B11P3Cu0.5C0.1 nanocrystalline alloy exhibits a high Bs about 200.6emu/g. The Bs of fully amorphous alloy Fe83.4Si2B10P4Cu0.5C0.1 drops dramatically to 172.1emu/g, which is lower than that of other nanocrystallines. Low material cost and excellent soft magnetic properties make the FeSiBPCuC alloys promise soft magnetic materials for industrial applications. •Partial substituting B by P helps to improve the glass forming ability of the alloy.•The addition of P content reduces the thermal stability and improves heat treatment temperature region for these alloys.•The Fe83.4Si2B11P3Cu0.5C0.1 nanocrystalline alloy exhibits a high saturation magnetic density of 200.6emu/g.