The SSE composite index comprises of all the A shares and B shares listed on the Shanghai Stock Exchange. The index can be tracked all the way back to Dec 19, 1990 and to a price of 100. Index trade volume is scaled down by a factor of 1000.
SSE Indices are all calculated using a Paasche weighted composite price index formula, similar to the CSI 300 Index. This means that the index is based on its base period for its calculations as compared to certain other exchanges, like the DAX, which base their calculations on the previous period (the laspyres price index calculation). 
Composite Indices and Sector Indices are weighted by shares.
Current index = Current total market cap of constituents × Base Value / Base Period
Total market capitalization = ∑ (price × shares issued)