Is it safe? If you do not control the access to your router than anyone close enough to get a signal can use your router with their phones/laptops, so it is a good idea to configure your router so only your devices can use them (mac addressing) and to secure the connection with encryption (WPA2).
Is it free? If you pay for your wi-fi bandwidth per gigabyte then the more data you transfer the more it will cost but where I live most people pay a fixed monthly charge for unlimited internet while most mobile/cellular rates are priced by GB. So here in the United States it is very beneficial to have our phones use our local wi-fi whenever possible to keep mobile/cellular bills lower but, I don't know how your internet is priced in China; if you pay per GB or have tier pricing then you will probably need to monitor your data usage.