please help me, how can I get transactions of a contract address in BSC network? what api?

an easy but expensive options would be this: https://www.bscscan.com/apis
(my bad there is a free plan)

This is a good option, there are some delays though I think. At least with free plan. Best bet that I know of would be to rent a node from a service provider and write code the query transactions. If you need mempool access it’s going to be more expensive