Conducted by the University of the Thai Chamber of Commerce's Center for Economic and Business Forecasting, the survey found the CSI rose from 39 points in the last survey in December to 46.
The closer a CSI score is to zero, the higher the level of corruption, while 100 shows high transparency and a lack of graft.
Centre director Thanavath Phonvichai said the survey of 2,400 respondents showed businessmen say they have paid fewer kickbacks to government officials and politicians at an average of 15-25% of project value, down from an average of 25-35% that they paid from 2010-13.
The amount means the country could save as much as 100 billion baht of state budget as every percentage point of reduction translates to 10 billion baht, according to Mr Thanavath.
"The reduction of 100 billion baht could edge up GDP by 0.5% to 0.7%," he said.
If the CSI performance continues to improve, Mr Thanavath is confident it will raise Thailand's ranking in the global corruption index produced by Transparency International Perceptions. Thailand now ranks 102nd after dropping from 88th last year.