China, who have failed to qualify for the 2014 World Cup finals, took an early lead over the Asian champions with a fifth-minute penalty.
Wang converted from the spot after Japan's Yuzo Kurihara brought down Yu Dabao.
Japan, without their established European stars, soon started to test China keeper Zeng Cheng.
They equalised in the 33rd minute when Kurihara made amends for giving away the penalty by turning in debutant Masato Kudo's header from a corner.
Coach Alberto Zaccheroni's men took control of the fast-paced match after the break and struck twice in the space of minutes around the hour mark to go 3-1 up.
Yoichiro Kakitani, playing his first match in national colours, deftly flicked in a cross and then his through pass found Kudo, who beat the advancing Zeng.
Wang, who also hit the post, brought China back into the match in the 81st minute by converting his second penalty before substitute Sun completed the comeback six minutes later by volleying in a cross from close range.
China will take on hosts South Korea, who drew 0-0 with Australia in the tournament opener, in their next match on Wednesday.
(Reporting by Sudipto Ganguly in Mumbai; editing by Justin Palmer)
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