1： If you configure the Opamp as a follower, the internal 1.024 which be output should also be 1.024v.
2： connect the resistor: 1024mv/20mA
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Please make sure that you connected resistor to the Opamp direct output pin (P3_6, P3_7, P0_0, P0_1). Direct pins have low resistance, while other pins may have 250-500 oHm resistance path from Opamap to the resistor. So the resulting voltage will be something like V = 1.024V x 1200 / (500 + 1200) = 0.723 V.
Keep in mind that even direct connection has some resistance, and Opamaps have some bias offset, so the output voltage will not be exact 1.024V.
You can find discussion of the same issue here:
KIT-059 annotation component: KIT-059: annotation component for CY8CKIT-059 PSoC5LP Prototyping Kit
Thank you very much odissey1. My problem is solved.