the credit spread option is used to hedge the devaluation of the underlying corporate bond, so if the underlying bond devalues, the option holder can receive the compensation. now what we need to do is to determine the compensation amount, right?
the compensation would therefore depend on the current value of the corporate bond, as the treasury rate, i.e. the risk free rate has increased to 6.3%, and the spread between risk free and corporate has mounted to 150bp, so the yield for the corproate bond should be 6.3+1.5.=7.8, and we use the 7.8 to determine the the price of the corporate bond, which is 948.95.
the strike price of option is determined at 961.40 using the yield at 7.6, which means if the price of the bond falls below the strike price, the option holder can get the compensation, equaling the difference between the two prices.
If the corporate bond has a much higher price, i.e. lowering spread and lower yield, the option holder would choose not to exercise the option.

什么都看不到？

the payout is determined by the difference between value of the underlying security  at the current spread and value of the underlying at strike spread.
please note to compute the PV, we use  N=18 instead of N=20 as one year has passed.
We need to value the bond with remaining semiannual payments for nine
years using two yields, y + S = 6.30 + 1.50 = 7.80% and y + K = 6.30 + 1.30 = 7.60%. This gives $948.95 and $961.40, respectively. The total payout is then
$50,000,000 × [$961.40 − $948.95]/$1,000 = $622,424.

refer to handbook fifth edition P558, example 22.11

yes. i found this example from handbook II. but i do not quite understand.
why the spead 150 is used to calculate actual price?