The probability that he chooses the winning box is 1/3, and the probability that the other two boxes win the prize is 2/3.

When the host removes a box that didn't win the prize, the probability of the remaining boxes winning the prize is still 2/3.

Therefore, if Zhang Kai changes his choice, the probability of winning the prize will increase.

(Actually, we can consider a more extreme situation: 100 boxes, and only one box wins the prize. When you choose a box, the host will take away the remaining 98 boxes, but there is no reward. Is the box in the host's hand easier to win the prize? Yes, it's 99/ 100)

1) Zhang Kai's winning probability is1/3 *1/2 =1/6.

2) The probability of Zhang Kai winning the prize is 2/3* 1/2= 1/3.