The job cut would amount to more than 20 percent of the company`s total workforce of around 26,000.
Wind turbine makers have been facing growing competition, putting pressure on pricing and inventory values and raising expectations for more takeovers to build scale.
The Spanish-based wind power joint venture, which cut its earnings forecasts twice in recent months, has to date been planning to cut only 700 positions.
But the company said on Monday it expects sales to fall to between 9 and 9.6 billion euros this year from about 11 billion in fiscal 2017, a 5-percent gain from year-earlier levels.
Underlying EBIT declined almost a fifth to 774 million euros in fiscal 2017 ended Sept. 30, though plunged by nearly two thirds to 192 million euros in the six months since April, it said.
"Our financial performance is still not at the level we`re all aiming for," chief executive Markus Tacke said.
"We believe that the planned restructuring measures at Siemens Gamesa are necessary because the market and regulatory environments are changing structurally," a spokesman for Siemens said.