To explore the effects of cardiac magnetic resonance imaging(CMRI) on the assessment of left ventricular function and myocardial changes in patients with obstructive sleep apnea-hypopnea syndrome (OSAHS). Thirty-one patients with OSAHS who were admitted to Handan Central Hospital from November 2019 to June 2021 were selected as the observation group, and 30 healthy volunteers who received CMRI monitoring during the same period were selected as the control group. Two sets of parameters related to left ventricular function were compared, including left ventricular ejection fraction (LVEF), left ventricular end diastolic/end systolic volume (LVEDV/LVESV), left ventricular myocardial activity imaging indexes[T1pre value(measurement of T1 relaxation time before enhancement) before mean injection of contrast agent in left ventricular myocardium, 15 min T1post value(T1 value of myocardial tissue measured by T1 mapping technique after using contrast agent in CMRI) and T2 value(transverse relaxation time of myocardial tissue)] after injection, and histological parameter characteristics of different myocardial layers [mean extracellular matrix volume fraction (ECV) of left ventricular myocardium]. The left ventricular function parameters (LVEF, LVED) in the observation group are significantly lower than those in the control group, while LVESV and T2 values are significantly higher than those in the control group(P < 0.05). There is no significant difference in T1pre, T1post, ECV in basal segment, middle segment and apical segment between the observation group and the control group (P > 0.05). CMRI has a significant clinical effect in patients with OSAHS and can effectively assess the left ventricular function, structure and early myocardial lesions.