Abstract: A total of 120 specimens of fly ash ceramsite lightweight aggregate concrete were designed for conventional triaxial compression tests in order to reveal the mechanical properties and behavior under complex stress conditions with ceramsite soaking time, strength level and lateral confining pressure as the changing parameters. The failure process and final failure behavior of fly ash ceramsite lightweight aggregate concrete under triaxial compression were observed, the stress-strain curves of specimens were obtained, and the influence of changing parameters on the mechanical performances was analyzed. The experimental results show that with the increase of the confining pressure, the failure behavior of fly ash ceramsite lightweight aggregate concrete changes from vertical splitting failure to oblique shear failure. When the confining pressure value is greater than 12 MPa, the specimen shows bulging failure without obvious cracks. The stress-strain curve is greatly affected by the value of confining pressure, but less affected by the immersion time and strength grade of the concrete. And after the confining pressure is greater than 12 MPa, the curve no longer has a descending section. The peak stress increases with the increase of ceramsite immersion time, strength grade and lateral confining pressure. The peak strain is not greatly affected by the soaking time of the ceramsite, it decreases with the increase of the strength grade, and increases with the increase of the confining pressure value. The elastic modulus increases with the increase of the strength grade and the confining pressure value, and is not significantly affected by the soaking time of the ceramsite.