Citigroup cut its target price on PTT Global Chemical Pcl to 68 baht ($2.13) from 80 baht ($2.51), citing uncertainty about changes in feedstock costs.
The broker, however, maintained its buy rating on the stock due to its cheap valuation.
PTT Global Chemical shares fell 2.24 percent to 54.5 baht by 0323 GMT. The stock has fallen around 10.3 percent since last Thursday.
"Market expectations that PTTGC's net back pricing contract with PTT would be revised led to sell-off pressure last week," Citi said in a report dated May 30.
There is uncertainty over when and how the new gas price contract would be changed, and the stock is likely to trade at a discount versus Asian chemical peers to reflect business risks faced by minority shareholders, the broker said.
1033 (0333 GMT)
(Reporting by Viparat Jantraprap in Bangkok; firstname.lastname@example.org)