Clinical efficacy in patients with cervical and upper thoracic esophageal cancer was compared between three-dimensional conformal radiotherapy (3D-CRT) and conformal intensity-modulated radiotherapy (IMRT). Medical data of 120 esophageal cancer patients undergoing RT in the Radiology Department in Xiaogan Hospital Affiliated to Wuhan University of Science and Technology (Xiaogan, China) from March 2015 to March 2018 were retrospectively analyzed. Patients were divided into 3D-CRT group (treated with 3D-CRT, n=60) and IMRT group (treated with IMRT, n=60). The target conformal index (CI), PTV maximum dose (D-max), PTV D-min and PTV D-mean in both groups were observed. Changes in double-lung V5, double-lung V20, double-lung V30 and spinal cord D-max in both groups were also observed. The serum free triiodothyronine (FT3), free tetraiodothyronine (FT4) and thyroid-stimulating hormone (TSH) concentrations were detected via chemiluminescence assay. Moreover, the short-term efficacy and radiation injury were observed. CI in IMRT group was significantly higher than that in 3D-CRT group (P<0.001). The organ-at-risk dose parameter V20 in IMRT group was obviously lower than that in 3D-CRT group (P<0.001). Serum FT3 and FT4 concentrations in both groups before RT were significantly higher than those after RT (P<0.05), but the TSH concentration was significantly lower than that after RT (P<0.05). Serum FT3 and FT4 concentrations in IMRT group after RT were obviously higher than those in 3D-CRT group (P<0.05). The incidence rates of radiation esophagitis and radiation pneumonitis in IMRT group were remarkably lower than those in 3D-CRT group (P<0.05). IMRT can reduce the double-lung V20 in patients, improve the target conformal degree, better protect the normal tissues, cause less damage to thyroid function, and reduce radiation injury during and after RT of esophageal cancer.