The Stock Exchange of Thailand (SET) is maintaining its average trading value target this year of 48-49 billion baht a day, though the level is likely to fall to 38 billion baht in the second half.
"The lower trading value has resulted from uncertainty in the US and political changes in many countries," said Pakorn Peetathawatchai, who heads the SET's corporate strategy and finance division.
Foreign investors are waiting for a clear policy from the US Federal Reserve on stimulus curbs, he said.
In July alone, the SET and the Market for Alternative Investment (MAI) had an average daily trading value of 45.94 billion baht, down 27% from a month earlier but up 64% from the same period last year.
Average daily trading for the first seven months stood at 59.188 billion baht, nearly double the volume of the same period last year.
Banks, energy and utilities remained the most popular with investors.
Mr Pakorn said shrinking trade value could hurt the initial public offering (IPO) market.
In July, companies listed on the SET and MAI raised a combined 9.45 billion baht in fresh funds, 5.34 billion of which was mobilised through IPOs and the rest through rights offerings and private placement.
For the first seven months, accumulated fund-raising via the equity market was 185.69 billion baht, up nearly 153% from the same period last year.
Apart from the falling trade value of foreign investors, that of local retail investors is likely to subside amid uncertainty from the local political situation and external headwinds.
"Investors' behaviour is also changing as they snap up big-cap stocks," said Mr Pakorn.
"Trading of the SET's 10 biggest stocks contributed 30% of daily trade value in July, up from 18% in March. The non-SET50 contributed 31% in July, down from 58% in March."
The SET's market capitalisation declined to 12.35 trillion baht in July, while the MAI's market cap rose marginally to 178.79 billion baht.