Fluor Corp., one of the world's largest engineering-construction and natural resources companies, yesterday announced an asset revaluation that will result in a one-time, $410 million write-off by the end of the calendar year.

The revaluation primarily affects Fluor's natural-resource holdings. But company officials said that does not mean Fluor is abandoning the minerals and metals industries. Instead, the company will continue to market its management expertise in those businesses, Fluor officials said.

Fluor's total losses for 1985, including the write-off, will amount to $600 million, company officials said yesterday.

Fluor lost $63.4 million, equal to 80 cents a share, for the first nine months of this year. The company's worldwide sales during the nine-month period totaled $3.1 billion.

The declining fortunes of Fluor, which earned $159.2 million in 1981, are attributable "to a worldwide recession in our two basic industries, natural resources and engineering-construction," Fluor spokesman Richard J. Maslin said yesterday.

Maslin said that Fluor's reevaluation of its assets is part of a strategy to facilitate their sale.

"It is primarily a financial thing. We're bringing their value down to realistic levels" dictated by the business environment in which the affected assets sit, Maslin said.

Fluor has been on a divestment binge recently, stripping itself of more than $800 million worth of holdings over the past year. Of that amount, an estimated $550 million involved Fluor real estate assets, and approximately $270 million concerned oil and gas properties.

The $410 million write-off announced yesterday will be in addition to those divestments.

Fluor's aggressive streamlining will "enhance" the company's core operations, Maslin said. "We're intensifying our efforts in engineering and construction," the spokesman said, adding that his company will try to win a big piece of the multibillion dollar market for new high-technology manufacturing plants.

The so-called "factories of the future" are being built in Canada, Japan, the United States and West Germany, particularly in the automobile industry.

China, Korea and a number of Latin American companies also are investing in the the computerized, roboticized plants, which are being designed to improve product quality and production efficiency.

Fluor officials said yesterday that the worldwide market for the high-technology factories could reach $100 billion by 1990.

On the natural-resources side, Fluor will concentrate less on its own mining and drilling and more on helping Third World countries to manage those kinds of operations, Maslin said. "We're not getting out of natural resources," he said.

Most of Fluor's 29,000 employes worldwide will keep their jobs through all of the restructuring, said Fluor Chairman David S. Tappan Jr.

Tappan said that a small number of Fluor employes will be encouraged to take early retirement and that some others will move to new jobs within the company. But, generally, Fluor employment will remain stable, Tappan said.