The power supply voltage of CN0269 is + 6V. If I use 5V power supply, will it affect the accuracy of sampling?
Using 5V instead of 6V to power the board should not affect the measurement, but it is still recommended to use the board as specified.
Looking at the schematic for the board found here it can be seen that the 6V voltage is being regulated to 5V and then down to 3.3V using ADP3336 and ADP121, respectively. Supplying only 6V to the ADP3336 could affect the 5V level and lower it to 4.6V in the worst case, according to the datasheet, but that voltage is not used on the board. The 4.6V level should still be enough to correctly regulate a 3.3V with the ADP121.
In conclusion, as stated at the beginning, the accuracy of the samples should be unaffected, but it is still best to use the board as intended.