BEIJING - China Life Insurance Co, the country' s biggest life insurer, said Monday its net profit for 2011 tumbled 45.5 percent year-on-year due to declining investment revenue and increased asset write-downs stemming from weak stock markets.

The company's net profit amounted to 18.33 billion yuan ($2.92 billion), according to China Life's annual report filed with the Shanghai Stock Exchange.

The company held one-third of the country's life insurance market last year, with premium income reaching 318.3 billion yuan, which was almost unchanged from a year earlier.

Investment return ratio stood at 3.51 percent last year, according to the report.

The company expects the insurance industry to face greater pressure and difficulty this year, as the slowing domestic and world economy will have an impact on its premium income, investment and capital replenish.