The invention discloses a method and system utilizing an ultrasonic reflection coefficient phase spectrum for continuous detection of whole scale lubrication film thickness. When a mutational zero crossing point appears in the reflection coefficient phase spectrum, frequency of the zero crossing point is extracted as resonant frequency for calculating the oil film thickness. When no mutational zero crossing point appears in the reflection coefficient phase spectrum, the oil film thickness is calculated according to the phase at the central frequency of an ultrasonic wave sensor. The inventionis characterized in that a problem of dead zone identification of a spring model method and a resonance method in prior ultrasonic detection is solved and all measurement ranges of a prior ultrasonicdetection model can be covered, so that continuous monitoring of oil film thickness of large scale range change is realized.