Honduran Emil Martinez grabbed a hat-trick and defender Zhou Ting added a penalty in front of a 60,000 crowd at the Workers' Stadium to win the capital club their first championship since a professional national league was formed in 1994.
"I'm really ecstatic. This day has taken Guoan more than a decade to achieve," China midfielder Tao Wei, who has spent 11 years at the club, told state media.
Beijing needed a victory to be sure of winning the title after starting the day level on points with Henan but with a superior goal difference.
Martinez settled their nerves with a goal after three minutes, beating the offside trap and scrambling the ball across the line at the second attempt.
The striker doubled Beijing's lead five minutes into the second half with a fierce left-footed drive from the wing and, after Zhou had made it three from the spot, claimed his hat-trick with a similar strike from closer range.
Beijing, owned by China's largest financial conglomerate and one of the best-funded clubs in the CSL, sacked South Korean coach Lee Jang-soo last month after suffering their first home defeat of the year.
"We've been through many difficulties to get to this point," said team manager Zhang Lu. "I was quite worried but it all turned out okay."
Defeat for Hangzhou, who lost 8-2 to Manchester United in a friendly in July, meant they took the second relegation spot in the 16-team league. Chongqing Lifan were already down.
Thousands of police were mobilised for the match in the Chinese capital after fans rioted on Thursday when tickets sold out but the game passed off peacefully.
To head off any chance of trouble, police issued a notice before the match banning any unapproved public celebrations around the stadium if Guoan won.
