Cooler Storage Ring (CSR) of Heavy Ion Research Facility in Lanzhou (HIRFL) consists of a main ring (CSRm) and an experimental ring (CSRe). Two particular C-type dipoles with embedded windings are used in the injection beam line of CSRm. They also act as the prototype dipoles of CSRe. The windings are designed to improve the field quality by their trimming current. The current impacts on field homogeneity and multipole components are investigated by a hall sensor and a long coil, respectively. The experiment shows that a field homogeneity of +/- 1.0 x 10(-3) can be reached by adjusting the trimming currents, though the multipole components change correspondingly. In our case, the quadrupole component is decreased to a low level with the octupole, decapole and 12-pole ones increased slightly when the trimming current is optimized.