The error signal used for stabilizing a laser frequency to an external bowtie-shaped ring cavity is analyzed theoretically based on the Hnsch-Couillaud frequency locking technique. In the experiment, the frequency of a 1583 nm fiber laser is locked to the longitude mode of a bowtie-shaped ring cavity with a KTP crystal inside it. The setup of this locking scheme is simple, and the technique is rather easy to operate, so it can be widely used in the laser technology, trace gas detection or other areas.