Fouling　on　heat　transfer　surfaces　causes　serious　and　economic　problems　in　industry.　Based　on　the　reaction　equations　of　the　dissociation　of　CaCO3,　an　expression　for　calculating　saturation　index　with　pH　value　is　proposed.　If　pH　value　is　higher,　the　saturation　index　is　bigger　and　the　crystallization　trend　is　greater.　Experiments　have　been　carried　out　for　the　pH　7-9　and　fluid　velocity　0.5-1　m/sec.　The　asymptotic　fouling　resistance　increases　and　the　induction　time　reduces　with　pH　value.　The　asymptotic　fouling　resistance　reduces　with　the　fluid　velocity.　The　influence　of　fluid　velocity　on　crystallization　fouling　is　greater　than　the　pH　value.