An symmetric encryption scheme with dynamic key based on obfuscation of point functions with multibit output(MBPF)is presented.A MBPF obfuscator with fully-entropic security, implying virtual black-box property(VBB), can be used to construct semantically secure encryption schemes with wrong key detection forα(n-weak keys. The symmetric encryption scheme is just the fully-entropic security MBPF obfuscator, whose key is deduced by a sensitive function . The sensitive function with two inputs, one is a secret key and another is a random key,and thus output key is random. The symmetric encryption scheme is more secure and implement simple.