The invention relates to a method for improving sensitivity of immune detection and particularly provides a biotin-avidin complex. The complex comprises (i) a trigger sequence; (ii) n-th level avidin, wherein n is a positive integer of between 1 and 20; and when n is 1, the 1st-level avidin is combined to the biotin on the trigger sequence; and (iii) a growth sequence, wherein the growth sequence is a single-strand oligonucleotide of which the two ends are marked by the biotin; the biotin at one end of the growth sequence is combined with the n-th level avidin, and the biotin at the other end of the growth sequence is combined with the (n+1)-th level avidin or is in an uncombined state; and n is a positive integer of between 1 and 20. The complex can effectively improve the sensitivity of the immune detection without basically changing or increasing the operation processes of the conventional immune detection test.