Low-energy protons are able to generate enough energy through direct ionization to cause a high single event upset cross section as the feature size of semiconductor devices shrinks. It poses a large challenge on the present proton single event modeling test technique and the space upset rate prediction method. Experimental study of proton single event effect in three different feature sizes of static random access memory (SRAM) (i.e. 65 nm, 90 nm, and 250 nm) is carried out based on domestic low-energy proton accelerators and also the foreign middle-high proton accelerators. Complete cross section curves of proton single event upset from low energy to high energy are acquired. Test results show that single event upset cross section below 1 MeV proton is up to three orders of magnitude higher than the saturation cross section of high-energy proton in nanometer SRAM. However, single event upset is not observed for protons below 3 MeV in 250 nm SRAM, and no single event multiple-cell upsets occur for protons below 1 MeV in 90 nm and 65 nm SRAM. The accurate geometrical structure model of composite sensitive volume is constructed through the combination of test data with device information, and calibrated further by single event test data of low-LET heavy ion and high-energy proton. Simulation results based on the model and Monte-Carlo calculation can reveal the root cause of low-proton single event upset cross section peak. Proton single event upsets are only caused through direct ionization of protons below 1 MeV. When low-energy protons pass through the multiple metallization and passivation layers of the device, the energy spectrum is broadened near the Bragg peak of the proton direct ionization, and the energy is deposited concentratedly into the sensitive volume through direct ionization. When the proton energy is too high or too low, the energy can not be deposited effectively into the sensitive volume through direct ionization. The energy spectrum straggling of low-energy protons due to the use of degrader has a large influence on the height and width of the single event upset cross section peak. Moreover, the contribution of low protons to the space proton single event upset rate is predicted for GEO orbit environment in the worst day environment. It shows that the direct ionization from low energy dominates the proton single event upset rate in the space in 65 nm SRAM. With the development of device technology, the critical charge of single event upset will be further reduced; and to the single event upset from low proton direct ionization more attention must be paid in the study and evaluation of single event effect.