Objective To investigate the relationship between the ratio of signal intensity of fluid-attenuated inversion recovery (FLAIR)sequence lesion side／contralateral side and the onset time of acute ischemic stroke. Methods Forty-three consecutive patients with acute ischemic stroke admitted to the Department of Neurology,Dongguan Hospital of Traditional Chinese Medicine from November 2016 to January 2018 were enrolled retrospectively. They were all first onset and were diagnosed by MRI,and the onset time was 0. 5 to 6. 0 h. According to the time window of effective rescue of the penumbra,43 patients were divided into a ≤4. 5 h group (n＝19)and a ＞4. 5-6. 0 h group (n＝24). The signal intensity values of FLAIR, diffusion-weighted imaging,signal intensity of apparent diffusion coefficient sequence on the lesion side and contralateral brain tissue in the two groups of patients were measured respectively,and the relative signal intensity (RSI)was calculated and compared between the two groups. Taking RSI as the dependent variable and onset time as the independent variable,a general linear regression analysis was performed. Results (1)In FLAIR sequence,the signal intensity value and RSI value of patients in the≤4. 5 h group were lower than those in the ＞4. 5-6. 0 h group. There were significant differences between the 2 groups (disease side signal intensity value:531 ± 109 vs. 681 ± 306,t ＝2. 04;RSI value:1. 19 ± 0. 13 vs. 1. 45 ± 0. 18,t＝5. 29,all P＜0. 05). There was no significant difference in the signal intensity values on the contralateral sides between the two groups (P＞0. 05). (2)In diffusion weighted imaging and apparent diffusion coefficient sequence,there were no significant differences in the signal intensity value and RSI value on the disease side and the contralateral side between the the≤4. 5 hours group and the ＞4. 5-6. 0 h group (all P＞0. 05). (3)In the FLAIR sequence,with the prolongation of the time of the disease,RSI gradually increased,and was positively correlated (r＝0. 756,P＜0. 01). Conclusions The quantitative measurement of the disease side／contralateral side of FLAIR sequence might reflect the onset time of acute ischemic stroke. Its RSI value increased with the prolonged onset time.