The invention discloses a passive power-based method and system for determining field loss protection of a phase modifier. The passive power-based method comprises the steps of performing And gate logic on first starting criterion, first low-voltage criterion and I-segment phase-entering passive power criterion, and performing And gate logic on second starting criterion, second low-voltage criterion and the I-segment phase-entering passive power criterion, wherein two And gate logic results and a OR gate logic form field loss protection I-segment criterion; and performing And gate logic on II-segment phase-entering passive criterion and excitation voltage criterion to form field loss protection II-segment criterion. When data information of the phase modifier conforms to the field loss protection I-segment criterion or the field loss protection II-segment criterion, the field loss protection reliably acts after a preset time threshold is delayed, the field loss protection is reliable without false action when the system is at a low voltage and the phase modifier is in the phase entering running process; and when a completely-demagnetized fault or a partially-demagnetized fault occurs in the phase modifier, the field loss protection acts reliability, and the problem of false operation of the field loss protection of the phase modifier when the system is at the low voltage and the phase modifier runs in a phase-entering way is solved.