Disclosed is detection apparatus for flow deformation of foundation layer in horizontal direction including: housing, rotation assembly rotatably disposed within accommodation cavity of the housing, and measurement assembly including first optical fiber lead wire, first optical fiber sensor disposed on first optical fiber lead wire, second optical fiber lead wire, and second optical fiber sensor disposed on second optical fiber lead wire, and disposed within the accommodation cavity. First optical fiber sensor is configured to measure tensile strain of first optical fiber lead wire and first optical fiber sensor before and after the rotation assembly rotates; second optical fiber sensor is configured to measure tensile strain of second optical fiber lead wire and second optical fiber sensor before and after the rotation assembly rotates, to obtain strain amount and displacement change amount, and further to obtain flow deformation degree and flow deformation direction of soil mass of the foundation layer.