Late goals by substitutes Rocky Neal and Niall Thompson made it six league and cup wins for Lee Hodges's team, who will be hoping for another home tie in Monday's draw.
Thoughts were heading towards a Tuesday night replay when City at last broke the deadlock with 17 minutes left.
The goal was set up by outstanding right wing-back Billy Palfrey who broke in from the right and his cross was bundled in by Neal, as last season's top scorer came off the bench to score for the second successive game after getting the winner in the midweek 1-0 victory over Chippenham Town.
Neal then turned provider four minutes from time, when his cross was tucked away from close range by Thompson for his first goal of the season.
City boss Hodges had named an unchanged team from the one that started. Tuesday's 1-0 National League South win over Chippenham Town, but as it was an FA Cup game had the luxury of being able to name seven substitutes, though only able to use three.
City began well and Tyler Harvey was only denied by a superb save from Portchester keeper Lewis Noice, who somehow pushed his glancing header from Billy Palfrey's right wing cross onto the post.
Noice was soon in action again as he tipped a powerful Ed Palmer free-kick over the bar.
The visitors also had defender Daniel Hayes yellow carded for bringing down Cody Cooke when the City striker looked to run clear.
The Hampshire visitors though also showed they were a goal threat, with striker Bobby Scott denied by City keeper Tom McHale after being put clear.
And later in the half, Scott headed tamely straight at McHale from a cross by right back Robert Evans.
City seemed to lose their way as the half progressed and it was the visitors who would have delighted to go in 0-0 at the break.
Ben Harding flashed a volley just wide for City at the start of the second half, while at the other end McHale tipped over a long range free kick from Portchester skipper Joe Bye.
And McHale had to be at his best soon after after when Robert Evans released Scott, with the home keeper deflecting his shot for a corner.
City continued to press, with Palmer having an effort cleared off the line before Neal finally got the vital breakthrough.
Noah Keats almost made it 2-0, but his shot hit the post and Portchester defender John Warren also struck the woodwork with a header in a frantic finish.
But Thompson then ended the visitors' brave resistance with City's second and they can now look forward to a third qualifying round tie and the the chance to make club history by reaching the fourth qualifying round for the fourth time.
Truro City: McHale; Gerring (capt), Palmer, Hartridge; Palfrey, Harding, Lamont (Allen 58), Keats, Riley-Lowe; Harvey (Thompson 84), Cooke (Neal 58).
Subs (not used): Todd, Yetton, Copp, Richards.
Yellow cards: Harvey (80)
AFC Portchester: Noice; Evans, Stone (Birmingham 77), Ramsey, Warren, Hayes, Searle (Baldacchino 80), Bye (capt), Scott, Kirby, Noakes (Mason 86).
Subs (not used): Wells, Tuppen, Gregory, Scott.
Yellow cards: Hayes (10), Stone (74)