The present application discloses a microchannel flat tube and a microchannel heat exchanger. The microchannel flat tube includes a flat tube body and a row of channels. The row of channels is arranged in the flat tube body along a width direction. The row of channels extends through the flat tube body along a length direction. A cross-section of each channel includes a first width in the width direction and a first height in a thickness direction. The row of channels at least includes a first group of first channels, a second group of second channels and a third group of third channels along the width direction. The first widths of the first channels, the second channels and the third channels decrease at a fixed rate, thereby facilitating the control of the thickness of the microchannel flat tube and improving the heat exchange efficiency of the third channels.