It has been reported that Everton made an offer of €25 million (£22.1m) plus €5m (£4.42m) in bonuses for the 25-year-old midfielder.
Watford, though, have turned down the bid from the Merseyside outfit, according to the report.
Current Everton manager Marco Silva worked with the former France Under-21 international when he was in charge of Watford from the summer of 2017 until January 21, 2018.
Doucoure has proved himself in the Premier League, and the Watford midfielder would be a good signing for Everton in the summer transfer window.
Doucoure - who earns £20,000 per week as salary, according to Spotrac.com - made 37 appearances in the Premier League for Watford last season, scoring seven goals and providing three assists in the process.