The Romanian has missed out on the chance to top the rankings on two occasions this year, but it was third time lucky as she beat Jelena Ostapenko 6-2 6-4 in the semi-final in Beijing.
Halep will dethrone Garbine Muguruza when the new rankings are released next week and becomes the third woman to take top spot since the middle of July following short spells for the Spaniard and Karolina Pliskova.
"It's very emotional, I think it's the first time I cried on court," Halep said. "It's amazing that I could do this. My team, everyone at home... is watching, and I want to thank everyone. It's my special day."
Garcia stuns world number one Halep to win China Open
Halep would have had the honour in June had she beaten Ostapenko in the French Open final, but the Latvian sprung a surprise at Roland Garros.
She got her revenge here, though, taking advantage of Ostapenko's weak second serve with two breaks in the first set.
Halep might have had a sense of deja vu when she found herself a set and a break up as that was the position of the French Open final when Ostapenko started a brilliant comeback and she threatened it again, as the Latvian rallied to lead the second set 4-3.
But Halep steeled herself and reeled off three successive games to become the 25th player in the history of the WTA rankings to sit at top spot.
Halep will face unseeded Caroline Garcia in the final on Sunday after the Frenchwoman beat 12th seed Petra Kvitova of Czech Republic 6-3 7-5 in the second semi-final.
Wuhan Open champion Garcia raced to her 10th consecutive win to climb into the world's top 10 as an error-strewn Kvitova recorded 11 double faults and won just 10 of 27 second serves.
The 23-year-old took a 3-0 lead in the first set to pile the pressure on Kvitova, who failed to make a comeback and lost the set.
Twice Wimbledon champion Kvitova started the second set strongly to gain a 2-0 advantage but her wayward serving cost her as Garcia levelled at 5-5 before clinching the match.
Garcia is on course to win her second consecutive title but Halep has beaten her in both their previous meetings.
Results from the China Open Women's Singles semi-final matches on Saturday
- Caroline Garcia (France) beat 12-Petra Kvitova (Czech Republic) 6-3 7-5
- 2-Simona Halep (Romania) beat 9-Jelena Ostapenko (Latvia) 6-2 6-4
Halep wins Kasatkina grudge match to reach China Open semis
Cirstea downs Pliskova to reach China Open quarter-finals