The present invention discloses ultrasonic nondestructive methods for pipe wall thickness measurement at high or low temperatures. An ultrasonic detection device comprises a first and a second ultrasonic waveguide. The waveguide length is selected according to the surface temperature of a pipe under inspection. A first piezoelectric plate causes generation of a plurality of ultrasonic excitation signals which is transmitted to the pipe through the first ultrasonic waveguide. The plurality of ultrasonic excitation signals has different group speeds when traveling along the first ultrasonic waveguide. The reflected ultrasonic wave signals are collected and transmitted to a second piezoelectric plate by the second ultrasonic waveguide. The pipe wall thickness is calculated using an ultrasonic wave signal which has the highest group speed. The first and second waveguides are arranged parallel and side by side. An isolation plate is disposed such that the first and second waveguides go through the plate perpendicularly.