1. A method for detection of nucleic acid methylation status, wherein said nucleic acid is double-stranded, said method comprises: (1) treating the nucleic acid double strands with a polymerase with 3′→5′ cleavage function or a 3′→5′ exonuclease, so that there is a deletion of 80 to 200 bases at the 3′ end of both strands; (2) adding dNTP, wherein cytosine (C) is methylated cytosine (5mC), so that the deletion at the 3′ end of both strands is end-filled and cytosine thereof is methylated cytosine; (3) treating the double strands in step (2), so that un-methylated cytosine is converted into uracil (U) while the methylated cytosine remains unchanged; (4) PCR amplifying and sequencing the nucleic acid to determine the methylation status of the nucleic acid, wherein the un-methylated cytosine has been converted into thymine in a portion of a sequence for determining a methylation site; the other portion of the sequence is the same as the original nucleic acid sequence because the cytosine has been methylated.