The invention provides a beam column connecting structure and a mounting method thereof, and belongs to the field of steel structures. The problem that an existing beam column is low in connection strength is solved. The beam column connecting structure comprises a circular column body and square beam body; a center pipe is arranged in the column body; the center pipe is fixed in the column body through a connecting piece; the center pipe comprises four central plates; the connecting piece is formed by bending a steel plate of a closed-loop structure; an inner partition plate is fixed in the center pipe; the center pipe is divided into inner pouring cavities through an inner partition plate; inner slurry passing holes are formed in the center plate; a square tube reinforcing tube is provided, and arranged in a space surrounding by the center plate and an outer connecting piece; the beam body comprises four beam plates and a square tube connecting tube located in the beam body; an outerpouring cavity is defined by the connecting tube and the beam body; the column body is provided with connecting holes; the beam body is arranged in the connecting holes in a penetrating matching mode; and the end of the beam body abuts against the outer side face of the center plate. The structure can improve the connection strength and stability of the beam column.