As a consequence of contracted and fenced grassland, the incidence of edema pathema in the Hequ horse is rising. The main pathological symptoms are edema, emaciation, anemia, heterophilia, inappetence, and dyskinesia. To study the cause of edema disease in Hequ horse, the Hequ horse farm with a high incidence was chosen as the experimental pasture, and the Azi husbandry experimental station without edema disorder was the control pasture. The research methods in this paper are included: (1) The mineral contents in soil, forage, blood, and liver were analyzed. (2) Routine parameters and biochemical values in blood were also measured. (3) Conduct a prevention trial and a treatment experiment. The results showed that Se contents in soil and forage was much significantly lower than that in the control group (P < 0.01), and there was no significant difference in other elements. Se contents in blood and liver in affected animals were very significantly lower than those of the control group (P < 0.01). Hb, HCT, MCV, and MCH were greatly significantly lower than those in the control group (P < 0.01). Activities of GSH-Px in blood were very significantly lower than those of the control group (P < 0.01). Before the onset season of the disorder in the affected area, NaSeO was used to conduct a prevention trial on 1576 Hequ horses. A dose of 0.03 mg NaSeO was given orally per kilogram of body weight, once every 15 days and twice continuously. There was no edema illness that year. In the treatment experiment, 235 horses were administered NaSeO orally at 0.04 mg per kilogram of body weight, once every 3 days for 4 consecutive times, and 198 horses were cured, with a cure rate of 84.26%. Therefore, it is possible that Hequ horse edema pathema is caused by Se deficiency in soil and forage.