A power supply circuit and a field emission electron source are provided. The power supply circuit includes: field effect transistors Si coupled in series via drains and sources in sequence, wherein 1≤i≤n, n≥2, and wherein a source of S1 is coupled to a negative electrode of a voltage source, and a drain of Sn is used as an output terminal of the power supply circuit to couple to a load; a first group of diodes D1i coupled in series; a first group of resistors R1j, 2≤j≤n; and a current feedback module configured to adjust an internal resistance of the field effect transistors Si, coupled in series in sequence, so as to cause a current passing through the load to be constant; wherein the field effect transistors Si, 1≤i≤n, operate in a constant current region.