State-owned ChemChina's first offer valued the agricultural chemicals group at 449 Swiss francs per share, or 41.7 billion Swiss francs ($41.72 billion), according to Bloomberg.
Citing unidentified sources, it said the rejection stemmed from regulatory concerns.
The two companies have not broken off talks and an agreement could still be reached in the next few weeks, Bloomberg said. Syngenta, under pressure to offer value to shareholders after turning down a big offer from Monsanto earlier this year, is also talking to other potential suitors, according to Bloomberg.
U.S.-listed shares of Syngenta were up 14 percent at $79.22 in extended trading on Thursday.
Syngenta and ChemChina could not immediately be reached for comment. Syngenta rejected Monsanto's cash plus stock offer in August. The offer was worth about $47 billion and Syngenta said it "significantly undervalued the company."
At the time, some shareholders expressed disappointment over the spurned deal and questioned the company's ability to improve its financial fortunes in a slumping commodity market.
To appease shareholders, Syngenta announced plans in September to buy back more than $2 billion of stock, funding the measure by selling its vegetable seeds business