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, 1≤i≤n, i and n are natural numbers, n≥2, and 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 i and j are natural numbers; and a voltage control module configured to adjust an output voltage of the voltage source to cause a current passing through the load to be constant; the field effect transistors Si, 1≤i≤n, operate in a resistive region.