The Shanghai Composite Index declined about 49% from its June 2015 peak to its January 2016 low.
The index peaked near 5,166 points on June 12, 2015, after a dramatic equity-market boom. By January 28, 2016, it had fallen to roughly 2,656 points. The resulting loss was approximately 48.6%, conventionally rounded to 49%.
The turbulence began when a heavily leveraged stock-market bubble burst. Many individual investors had borrowed to buy shares, magnifying both the rise and the subsequent losses. Government measures—including trading halts, restrictions on selling and efforts to support prices—slowed but did not immediately stop the decline.
A common mix-up is citing only the first stage of the sell-off: the Shanghai index lost about 30% in the first three weeks after the June peak. The later summer sell-off and renewed weakness in January 2016 produced the much larger peak-to-trough figure. The episode was severe for equities, but it did not cause the economic collapse some commentators predicted.