Beijing's population fell for the first time last year, and the capital still experienced a low fertility rate despite the introduction of the two-child policy in 2016, according to a recently published blue book.

The blue book says the number of permanent residents in Beijing was 21.7 million by 2017, a 22,000 decrease year-on-year. Among them, the number of permanent migrants fell by 132,000 to 7.9 million, while the number of registered residents dropped by 37,000 to 13.6 million, the Beijing News reported on Sunday.

The blue book, titled Beijing's Population Development and Study Report 2018, was published by the Beijing Population Research Institute at Party School of the Communist Party of China (CPC) Beijing Municipal Committee and Social Sciences Academic Press on Sunday.

Beijing's birth rate rose from 7.96 per thousand in 2015 to 9.06 per thousand in 2017, but remains very low.

Beijing's birth rate dropped below 10 per thousand in 1991, which is considered a super low fertility rate in demography, according to the blue book.

Independent demographer He Yafu told the Global Times on Monday that the drop in the migrant population in Beijing was the result of its city plan for 2016 to 2035 which plans to cap its population at 23 million by 2020.

The drop of the registered population was mainly caused by its continuing low fertility rate, He said.

The statistics show that the effect of the two-child policy was not obvious in Beijing, as well as other cities in China, because of the high cost of raising children, He said.

"Even if Beijing starts to encourage births, the low birth rate would not improve much. Thus, to ease the severe aging problem, the Chinese capital should relax its restrictions on hukou, or household registration, to attract more talent and migrants," He noted.

He said migrant workers of mostly working age have greatly eased Beijing's aging process.

Meanwhile, the blue book also states that Beijing's aging problem is getting severe, with the number of residents aged 65 and over increasing from 1.7 million in 2010 to 2.4 million in 2017.