Catalysis of the reaction: S-adenosyl-L-methionine + (S)-7,8,13,14-tetrahydroprotoberberine = S-adenosyl-L-homocysteine + cis-N-methyl-(S)-7,8,13,14-tetrahydroprotoberberine.
tetrahydroprotoberberine cis-N-methyltransferase activity