Why is the contract signed by AVIC Property outsourced?

According to what you said, the property management company intends to outsource the security work to other companies, which is completely possible and legal commercial operation. However, if you sign a labor contract with the property management company, the property management company cannot dismiss you at will before the contract expires. If the property management company dismisses you before the expiration of the labor contract, it should compensate you instead of simply dismissing you. If the property management company doesn't compensate you, I suggest you ask the local property management department or the labor arbitration institution for help.