According to the principles of harmonic-based earth fault protection（identification to the line with a single phase to earth fault） in non-solidly earthed network， the characteristics of non-linear load， system power， fault point as a harmonic source and the faulty harmonic current generated by them are analyzed. Through harmonic-based protection system， the influences of multi-harmonic sources are studied as well. It is discovered that the harmonic sources of system power and non-linear load play dominant roles which give their effect through the pre-fault harmonic voltage at the fault point. The influences of fault point can be neglected for it produced neglected harmonic current. The magnitude and phase of harmonic sources and the distance between fault point and harmonic sources are the main factors affecting the reliability of harmonics-based protections. The analysis results are proved by digital simulation.