The invention provides a sea ice thickness measuring apparatus which comprises a body. The body comprises: a sonar device which transmits and receives sonar signal perpendicular to the seawater or sea ice; a laser which transmits and receives laser signal perpendicular to the seawater or sea ice; and a control unit which is in data connection with the sonar device and the laser; wherein, the control unit calculates the first height between the body and the sea ice upper surface according to the sonar signal and the laser signal, calculates the second height between the body and the seawater surface according to the sonar signal and the laser signal, and obtains the ice side height by subtracting the first height from the second height, then calculating the sea ice thickness by using the archimedes law. The invention further provides a sea ice thickness measuring method. The invention realizes the repeatable observation of large-scale sea ice, and can measure the sea ice thickness accurately.