Fast chlorophyll fluorescence kinetics were measured by a PEA, Plant Efficiency Analyzer from Hansatech Instruments, in two strains of white cucumbers (Cucumis sativus L.) with short-fruit-type or long-fruit-type. The goal is to analyse the chlorophyll a fluorescence kinetics and the thermodynamics of photosystem Ⅱ (PSⅡ) heat shock stability of two contrast varieties of ‘XINTAIMICI’ with lower heat resistant temperature and ‘JINCHUN Ⅳ’ with a higher one. Along with increasing temperature (5 min at 30-57 ℃), the structure and function of PSⅡ shows to be more sensitive to heat shock as maximum quantum yield of primary photochemical of PSⅡ(Fv/Fm=1-Fo/Fm). The density of reaction centers (RC/CS) and the activity of the oxygen-evolving-complex activity (ρk) show a sigmoid decay, while in contrary energy dissipation per RC(DIo/RC) increases. The analysis of the data show that the determinant mechanism under the heat shock, i.e., the reversible deactivation of the PSⅡ reaction centers and of oxygen-evolving-complex (OEC) as well as the energy dissipation, plays a vital role in protecting PSⅡ to prevent photoinhibition. For heat shocks above 30 ℃ the ρk starts to decrease. Over 40℃, RC/CS begins to decrease and over 44℃, DIo/RC increases. Over 51 ℃, in ‘XINTAIMICI’ high temperature weakens the capacity of the dissipative protection mechanisms of PSⅡ.The denaturising midpoint temperature (Tm) calculated from Gibbs free energy (ΔGD ) indicates that the heat stability of protein complexes is higher than that of reaction center or that of oxygen-evolving-complex (OEC). Each midpoint temperature Tm for Fv/Fm, RC/CSo or ρk shows that the thermotolerance of ‘JINCHUN Ⅳ’ is the highest among the four cultivars. The white cucumbers with short or long fruits follow afterwards and ‘XINTAIMICI’shows the lowest heat shock resistance.