For the issue of defaults in L2 morphology， McCarthy proposed Morphological Underspecification Hypothesis．This paper introduces the MUH， thinks about its contributions and shortcomings， and tests it by a study on the gender and number in spoken Spanish by Chinese Spanish learners． The results support the assumption of MUH that defaults are underspecified forms， and L2 learners produce primarily underspecification errors， but contradict its prediction that learners avoid feature clash errors．