The characteristics of the transverse modes of a sealed-off He-N2-CO2 laser are measured on-line by using a laser beam analyzer. The influence of the transverse modes on discharge current is observed. It is shown that this laser tube can easily operate on lower order modes under optimum discharge current，but hardly operate on the TEM00 mode. Under the condition of improved symmetry of laser tube and increasing the speed of cooling water, the laser can operate on the TEM00 mode. The variety of laser modes is analyzed through the kinetic process theoretically, which is helpful to the control and improvement of laser mode for high power lasers.