The invention discloses a single-way downpipe fixing clamp and belongs to the field of building hardware. Problems of complicated operation and looseness due to long term use of traditional downpipe clamps during the use can be solved. The single-way downpipe fixing clamp comprises an external clamp, fish-shaped locking strips, an internal clamp and a fixing rod, wherein the external clamp and the internal clamp are both in a semi-circle structure; two ends of the external and internal clamps are both provided with a side-ear structure; each side-ear structure radially extends; a square through hole is formed in the middle part of each side-ear; the end surface of each fish-shaped locking strip is provided with a square projection; triangular projections are evenly distributed on two sides of each fish-shaped locking strip along a length direction; a cross section of each fish-shaped locking strips is the same as through holes of the side-ears of the external and internal clamps; the fixing rod is welded on the center position of an external cylindrical surface of the internal clamp; and the fixing rod is radially distributed. The single-way downpipe fixing clamp is simply structured and can be simply operated; and the downpipe can be well fixed.