The invention provides alginate lyase, host cells capable of secreting alginate lyase and application of the alginate lyase and the host cells. The alginate lyase has any one of amino acid sequences shown as (I) and (II): (I) an amino acid sequence shown as SEQ ID NO. 1; (II) an amino acid sequence obtained by carrying out substitution, deletion or addition of 1 to 20 amino acid residues on the amino acid sequence shown as SEQ ID NO. 1; the obtained amino acid sequence has the activity of the alginate lyase. According to the alginate lyase, whole genes of the alginate lyase are truncated to reduce the molecular weight, so that the expression amount of the host cells on the alginate lyase is remarkably improved; the secreted alginate lyase has high activity, good stability and strong degradation capability on sodium alginate.