1. It is obtained by the reaction of formaldehyde with diethanolamine. Add formic acid into the reaction pot, heat it to boiling, drop the mixture of formaldehyde and diethanolamine under stirring, drop it for about 1H, maintain the temperature at 90-98 ℃, and continue reflux for 4H. Then carry out vacuum distillation and collect the fraction of 120-130 ℃ (0.53kpa) to obtain N-methyldiethanolamine with a yield of 85%. 

2. It is obtained by the reaction of methylamine with ethylene oxide. Keep the temperature below 30 ℃, pass ethylene oxide gas into 20% methylamine solution for addition reaction until the relative density of the reaction solution reaches 1.025. The end point is stirring for 15min and the relative density remains unchanged. Recover methylamine to 103 ℃ under normal pressure, evaporate water under reduced pressure, and collect 119-170 ℃ (4.67kpa) fraction. It is the finished product. The yield was 72%. Raw material consumption quota: ethylene oxide 950kg / T, methylamine (40%) 870kg / T. In addition, N-methyldiethanolamine can also be prepared from formaldehyde and cyanoethanol by catalytic hydrogenation.