Cnooc Ltd. is likely to slash spending further after low oil prices led to a sharp drop in the Chinese state-controlled oil giant’s first-quarter revenue.

The partial results released on Friday by the listed unit of state-owned China National Offshore Oil Corp. indicate further pain ahead for the Chinese oil industry as it grapples with a price slump that is challenging energy companies around the globe, as well as softer demand at home.

Cnooc has pledged to cut spending by roughly one-third compared with a year ago, but its figures indicate it has more cutting to do to reach that goal.

“Capital expenditure was reduced significantly,” said Neil Beveridge, analyst at Bernstein Research. “It needs to be cut more to achieve the full-year guidance.”

Cnooc Chief Executive Li Fanrong praised the company’s performance under what he called “harsh circumstances,” but pledged greater efforts throughout the rest of 2015.

“We will continue to strengthen our internal operations management, exercise strict cost control and enhance efficiency to proactively respond to the impact of low oil prices,” Mr. Li said in a statement.

First-quarter revenue fell nearly 40% to 36.72 billion yuan ($5.93 billion) from 60.46 billion yuan a year earlier because of falling energy prices. Cnooc said realized oil prices—the average price it earns for each barrel sold—fell 49% in the first quarter versus the same period last year. Spending in the first quarter fell about 16%, led by a 35% drop in exploration costs.

The company didn’t disclose net profit for the first quarter.

A lower cost base versus Chinese rivals such as PetroChina Co. and Sinopec Corp. has helped Cnooc better weather the fall in oil prices. The company’s 2014 net profit rose 6.6% even as oil prices plunged.

But plummeting energy prices are testing Cnooc’s ability to maintain what has been rapid production growth in recent years, while maintaining the aggressive cost controls that made it a favorite of investors. The company’s shares in New York have risen about 25% so far this year.

Cnooc said net oil-and-gas production growth slowed in the first quarter to 9.4% year-over-year to 118.3 million barrels of oil equivalent, compared with nearly 15.5% growth last year. Overseas production growth was flat, held down by falling output at its Canada operations.

Like oil companies everywhere, Cnooc is being forced to adjust to swift changes in global energy markets. Rapid oil-and-gas production out of the U.S. and elsewhere has flooded global markets with new supplies. Meanwhile, a slowing economy has been suppressing demands at home for the major Chinese oil companies.