By setting several intercropping patterns of cassava with peanut, we used the LI-6400 photosynthesis system to study the diurnal variation of photosynthesis under different treatments. The results showed that: ①The diurnal variations of photosynthesis of both cassava and peanut had a single peak: the curves of PAR and atmospheric RH open downward when the curves of Ca and Ta open to a opposite direction. ②During several intercropping patterns, narrow-intercropping had no intercropping advantage while wide-intercropping seems to be an optimal production program. However, the WUE and Ls of wide-intercropping were the lowest which would make it not popular in the arid region; Double-intercropping had an efficient use of water and CO2 which would play an important role in the area shortage of resources. ③Four factors had significant effects on the Pn of both peanut and cassava, namely, Tr, Ci, Cond and Ls. Furthermore, Tr, Cond and Ls had positive correlation with Pn while Ci showed negative correlation. Peanut and cassava had "midday depression" by which stomata mechanism was impacted by both transpiration rate (Tr) and intercellular CO2 concentration (Ci), which meant that the mechanism of photo inhibition worked first and then followed by the mechanism of Ci.