Objective To explore the effect of preoperative hypothyroidism on postoperative cognition dysfunction (POCD) in elderly patients after on-pump cardiac surgery. Methods Patients who were no younger than 50 years and scheduled to have on-pump cardiac surgeries were selected in West China Hospital from March 2016 to December 2017. Based on hormone levels, patients were divided into two groups: a hypo group (hypothyroidism group, thyroid stimulating hormone (TSH) >4.2 mU/L or free triiodothyronine 3 (FT3) <3.60 pmol/L or FT4 <12.0 pmol/L) and an eu group (euthyroidism group, normal TSH, FT3 and FT4). The mini-mental state examination (MMSE) test and a battery of neuropsychological tests were used by a fixed researcher to assess cognitive function on 1 day before operation and 7 days after operation. Primer outcome was the incidence of POCD. Secondary outcomes were the incidence of cognitive degradation, scores or time cost in every aspect of cognitive function. Results No matter cognitive function was assessed by MMSE or a battery of neuropsychological tests, the incidence of POCD in the hypo group was higher than that of the eu group. The statistical significance existed when using MMSE (55.56% vs. 26.67%, P=0.014) but was absent when using a battery of neuropsychological tests (55.56% vs. 44.44%, P=0.361). The incidence of cognitive deterioration in the hypo group was higher than that in the eu group in verbal fluency test (48.15% vs. 20.00%, P=0.012). The cognitive deterioration incidence between the hypo group and the eu group was not statistically different in the other aspects of cognitive function. There was no statistical difference about scores or time cost between the hypo group and the eu group in all the aspects of cognitive function before surgery. After surgery, the scores between the hypo group and the eu group was statistically different in verbal fluency test (26.26±6.55 vs. 30.23±8.00, P=0.023) while was not statistically significant in other aspects of cognitive function. Conclusion The incidence of POCD is high in the elderly patients complicated with hypothyroidism after on-pump cardiac surgery and words reserve, fluency, and classification of cognitive function are significantly impacted by hypothyroidism over than other domains, which indicates hypothyroidism may have close relationship with POCD in this kind of patients.