[Objective] To use Platelet Additive Solution Ⅲ M to suspend concentrated platelets and evaluate their quality at different storage periods, in order to investigate the optimal ratio of Ⅲ M to plasma in the medium for storing concentrated platelets. [Methods] Disposable plastic blood bags with platelet storage bags were used to collect whole blood from healthy voluntary blood donors, and concentrated platelets were collected by plasma-rich method, with a volume of about 50 mL and ≥4.0×10¹⁰ platelets contained in each bag. According to the Platelet Addictive Solution ⅢM/plasma volume ratio in the medium of suspended platelets, the platelets were divided into 3 groups: control group (plasma only), experimental group 1(Platelet Addictive Solution ⅢM/plasma volume ratio of 6.5∶3.5) and experimental group 2 (low plasma group, Platelet Addictive Solution ⅢM/plasma volume ratio of 9∶1), each group of 50 samples. Three groups of platelets were stored at (22±2) ℃ at a flat-bed shaker, and 5 mL were sampled by sterile connection at day 1, 3, 5 and 7 respectively to detect platelet count, pH value, lactate dehydrogenase, CD62P positive rate and Annexin V positive rate. All the data were analyzed with SPSS24.0 software. One-way ANOVA was employed to compare the differences among three groups. In order to pairwise comparisons between means of multiple samples, Bonferroni method was applied. [Results] With the extension of storage time, the platelet count decreased and the Annexin V positive rate increased in the 3 groups, and the difference of the 3 groups was not statistically significant (P>0.05). The pH value decreased in the 3 groups, with values at day 1, 3, 5 and 7 of 7.44±0.13 vs 7.44±0.14 vs 7.41±0.11, 7.31±0.68 vs 7.43±0.23 vs 7.22±0.12, 7.30±0.15 vs 7.42±0.14 vs 7.17±0.12, 7.29±0.33 vs 7.26±0.18 vs 7.04 ± 0.12, respectively. The pH decline in the control group and experiment group 1 was minor, with no statistically significant difference, but experiment group 2 showed relatively larger decreases in day 5 and day 7, with statistically significant difference (P<0.05). LDH concentrate was elevated in 3 groups (mmol/L), with values at day 1,3,5 and 7 of 169.62±99.33 vs 105.80±150.71 vs 77.14±105.38, 225.10±112.86 vs 116.00±72.77 vs 94.42±88.74, 249.42±79.55 vs 119.00±53.51 vs 118.35±80.39, 253.34±86.95 vs 147.71±90.71 vs 124.68±128.68 respectively. Compared with the control group, the difference was statistically significant (P<0.05). Experimental group1 had the smallest increase; CD62P positive rate increased in 3 groups (%), with values at day 1, 3, 5 and 7 of 26.22±11.74 vs 23.48±12.48 vs 40.49±11.86, 41.29±8.36 vs 33.53±25.64 vs 50.42±22.36, 59.59±10.13 vs 36.39±23.10 vs 50.94±20.50, 72.92±15.44 vs 55.54±23.65 vs 61.89±18.82 respectively. Compared with the control group, the increase in experiment group1 was smaller, and the difference was statistically significant (P<0.05). [Conclusion] Platelet Addictive Solution ⅢM/plasma volume ratio of 6.5∶3.5 is superior to traditional plasma in maintaining platelet quality during the in vitro preservation period of platelets.