Abstract: A composite caisson is one of preferred foundations for transmission-line engineering in soft soil areas. However, the application of composite caissons in soft soil areas is restricted owing to lack of full understanding on bearing characteristics in soft foundation. In order to study the horizontal bearing characteristics of composite caissons in soft foundation, field tests of foundations subjected to horizontal forces at different heights and with different caisson lengths were conducted. The horizontal bearing characteristics, such as base displacement, rotation angles, locations of rotation centers, earth pressure on foundations, etc were analyzed. The main conclusions were concluded:to increase caisson lengths could reduce the maximum additional earth pressure on foundations, but barely improve the horizontal bearing capacity of foundations. The earth pressure on foundations was in a bilinear form, and the zero-pressure points were near geometric axes of foundations. The resultant forces of earth pressure on foundation bases were not zero, which could produce additional axial tensile forces on caisson foundations.