Interlanguage fossilization is normal for second language acquisition. It is also a hot for studies on theory of foreign language acquisition. Many reasons cause the interlanguage fossilization. This paper probes into the internal mechanism of interlanguage fossilization from five aspects, namely the physiological aspect, the psychological aspect, the cognitive manner, the cultural difference, and the cognitive emotion. In a perspective of teaching, this paper advances possible solutions for overcoming the fossilization phenomenon in foreign language teaching.