This study was designed to observe the negative pressure in the cup during cupping, and to investigate the influence of negative pressure on the depth of filiform-needle inserted and retained. In the beginning, the change of pressure after cupping on acupoint BL23 for a span of 20 minutes was recorded. Acupuncture on acupoint BL23 was performed; the filiform-needle was retained and followed by cupping at different levels of negative pressure, and the changes of the depth of needle insertion was measured. The results showed: the absolute value of the negative pressure in the cup goes down with time and is significantly lower after 20 minutes (P < 0.05); the negative pressure in the cup causes the retreat of the inserted needle; and under the same pressure, there is a greater length measurement in needle retreat for male than for female (P < 0.01). The length of needle retreat is related to the pressure in the cupping-cup and to gender.