Objective To explore the clinical value of skeletal muscle ultrasound gray scale (GS) values and muscle thickness (TH) in the quantitative diagnosis and post-discharge functional status evaluation of patients with intensive care unit-acquired weakness (ICU-AW). Methods A total of 99 ICU patients from Shanghai Fourth People’s Hospital Affiliated to Tongji University were enrolled. According to the Medical Research Council scale (MRC-ss), patients were assigned to ICU-AW group (n=52) or non-ICU-AW group (n=47). MRC-ss scoring and ultrasonography were performed on days 1, 3, and 7 after ICU admission, respectively, and the GS and TH of the bilateral biceps brachii and rectus femoris were measured. The diagnostic performance of GS and TH for ICU-AW was evaluated using receiver operating characteristic (ROC) curve analysis. Additionally, the Barthel index (BI) at 3 months after discharge was used to evaluate the relationship of GS and TH with post-discharge functional status. Results In the ICU-AW group, both GS and TH values progressively declined with the duration of ICU stay. By day 7, GS values of both the biceps brachii and rectus femoris were significantly lower in the ICU-AW group than in the non-ICU-AW group (both P＜0.001). Rectus femoris TH was also significantly reduced (P＜0.01), while no significant difference was observed in biceps brachii TH between the 2 groups (P＞0.05). Pearson correlation analysis indicated that biceps brachii GS, rectus femoris GS, and rectus femoris TH were positively correlated with MRC-ss scores (all P＜0.05). ROC curve analysis showed that the area under curve (AUC) values of biceps brachii GS and rectus femoris GS for diagnosing ICU-AW were 0.782 and 0.761, respectively, which were superior to the diagnostic performance of biceps brachii TH (AUC=0.566) and rectus femoris TH (AUC=0.711). When biceps brachii GS and rectus femoris GS were combined, the diagnostic AUC improved to 0.896. After the patients were grouped by the ROC cutoff values for diagnosing ICU-AW, BI scores were lower in the low-GS groups of both biceps brachii and rectus femoris, though the differences were not significant (both P＞0.05). No significant difference in BI scores was found between the high and low biceps brachii TH groups (P＞0.05), whereas the low rectus femoris TH group had significantly lower BI scores than the high rectus femoris TH group (P＜0.01). Conclusion GS and TH measurements of the biceps brachii and rectus femoris may serve as quantitative ultrasound indicators for ICU-AW assessment. GS shows better diagnostic performance than TH, and the combined use of biceps brachii GS and rectus femoris GS further improves the diagnostic performance for ICU-AW. Rectus femoris TH also provides a useful reference for evaluating post-discharge functional status.