不一定要交税 ，如果你invest 自己的钱超过旧房子的mortgage balacne

If the Relinquished Property has a mortgage on it, then the relief from that mortgage debt (when it's paid off in the sale) is considered mortgage boot to you UNLESS: 
(a) the Replacement Property is mortgaged for an equal or greater amount of debt, OR 
(b) you invest your own money towards the purchase of Replacement Property to offset the

debt relief
• on the property sold.