The present invention discloses a method for characterizing the State of Charge (SOC) of lithium-ion batteries using an ultrasonic reflection coefficient, which employs a water immersion ultrasonic detection method for measuring the reflection coefficient angular spectrum of lithium-ion batteries. This invention pertains to the field of non-destructive testing technology. A pouch-type lithium-ion battery can be regarded as a laminated structure composed of multiple materials, and when the State of Charge (SOC) of the lithium-ion battery varies, its reflection coefficient changes accordingly. The invention acquires the reflection coefficient angular spectrum of lithium-ion batteries at different SOCs through ultrasonic water immersion detection, establishes a mapping relationship between the angular spectrum and the SOC of the lithium-ion battery, and uses the distance between the two peak values of the angular spectrum to characterize the SOC of the lithium-ion battery. The invention enables non-destructive characterization of the SOC of lithium-ion batteries and allows for localized SOC measurement of the batteries.