Those killed include 12 firefighters. The toll is likely to shoot up, China's official Xinhua news agency said on Thursday.
Two blasts happened in a warehouse storing "dangerous and chemical goods" in the port city of the area.
A huge fireball lit up the sky, and nearby buildings collapsed.
President Xi Jinping has urged "all-out efforts" to rescue victims and contain the fire.
According to Xinhua, the first explosion took place at about 23:30 local time (15:30 GMT) on Wednesday and was followed by a more powerful blast seconds later.
State broadcaster China Central Television (CCTV) said a shipment of explosives had detonated but this has not been confirmed.
Senior managers of Ruihai Logistics, which owned the warehouse where the explosion took place, are being questioned by authorities, state media reports.