The index declined 1.1 percent from a month before to 236.59 at the end of May, said CISA, which forecast iron ore prices to stay relatively stable in the coming months.
In a breakdown, the homegrown iron ore price index was up by 0.57 percent month-on-month to 227.59, while the imported iron ore price index went down 1.34 percent to 237.95 at the end of May.
Due to the impact of environmental inspections, demand expectations in the iron ore market went down slightly, while iron ore stocks at ports went up slightly, the association said.
By the end of May, imported iron ore stocks at domestic ports amounted to 160 million tonnes, up 0.83 percent month-on-month, and up 14.35 percent year on year, according to CISA.
In the first four months this year, China imported 353 million tonnes of iron ore, almost the same as the same period last year.
As demand is getting weaker and supply still exceeds demand, it will be hard for iron ore prices to rise significantly, CISA predicted.